Yes. I was clear on that. Mutantum basically allows you to start with some spells or invent others that have equivalents to the Boosted, Harnessed and Tethered masteries. He needs Tame magic to use Harnessed and Tethered in Spontaneous spells.
The entire point that I spoke up from the gallery was that he needs Mutantum Magic as a prereq to buying Tamed Magic.
I have Mutantum Magic and won't be getting rid of it. Peregrine said I need to stick with Puissant Creo/Muto as my House virtue, so I'm swapping out Affinity with Muto for Puissant Muto, making that my free virtue, and buying Mutantum normally. Sorry if that wasn't clear.
The more I look at this, the less I want to give up Affinity with Muto. Is there any objection to me burning a season to initiate Vulgar Alchemy as a mystery virtue? I don't have a specific cult in mind, I just know I'd like to pursue Hermetic and Philosophic Alchemy in the future to support Antoninus's role as a House enchanter. Provided the enormous piles of vis on-hand don't render them moot, that is.
This post is long, because it's going into some deep analysis, and asking you some questions which might help you figure out what you want from the character.
There's a difference being the House's go to Enchanter, and the House's go to Enchanter for specific items/effects. Let's face, it, I'd think it's pretty likely that House Mercere turns to House Verditius for a lot of enchantments. Now, can Antoninus fill a niche that a Verditius doesn't? Certainly and totally possible and reasonable. Niches can be very profitable for the individual.
Philosophic Alchemy, when you are looking at it to augment your MT score to try and handle more vis in a season isn't all that great. It's governed by your CrVi Extraction total, and Creo and Vim aren't Antoninus's specialties. Let's look at his CrVi Extraction total (Cr 8 + Vi 7+ Int 2 + MT 5 + Aura 3) of 25. That's 3 pawns. He can move to Eilon Chon, if the lab is available and add 12 to the total for a total of 4 pawns. Getting to 5 isn't all that far off. Let's say that Philosophic Alchemy adds 5 pawns to just round it out.
Let's run this further out, and move your MT score up to 6, since we have a summa and you can get it to that level in a couple of seasons with a Q15 book. Let's also say that you've acquired Major Philosophic Alchemy. And also let's say that you change your specialty for MT to Enchanting, making it an effective 7. So now, for the bulk of Antoninus's career he can effectively handle 19 (5 from Philosophic Alchemy and 14 from an effective MT of 7) pawns of vis when opening a device, working diligently he can probably get that up to the mid 20s by the end of his career, but probably not much further than that. Having an able assistant will push his CrVi extraction total up, having a customized lab will help, but when you're dividing a CrVi Lab total by 10, it's not a lot of vis that you can add. And, to top it off, it requires an extra season of work and some pre-planning/effort. So for 2 seasons of effort, you can probably handle opening a device with 19 pawns of vis. Yes, you can do something else during that season where you're pre-staging 5 pawns of vis...still requires the planning.
Vulgar Alchemy has some problems and was recently discussedon the main Ars forum. It certainly doesn't add much, IMO, and is definitely unwieldy as written. I don't believe Vulgar Alchemy is at all necessary to initiate Hermetic Alchemy. Why not skip to Hermetic Alchemy, unless there's something in Vulgar Alchemy that you really want? And I personally don't have an issue with you initiating Hermetic Alchemy or Vulgar Alchemy with some kind of sacrifice. I think you were on this path because of The Order of the Green Cockerel, perhaps? I'm recalling that from memory, so I might be wrong.
All that being said, Talia is very interested in these Mysteries (or will be when she hears about them), too, so if Antoninus is interested in having an amica following a similar path, Talia is one to be there. And I wouldn't mind, personally, as a player, developing and/or exploring a mystery. Talia does want to enchant a ship, but she has some advantages over Antoninus, four to be precise: Puissant Creo, Affinity with Creo, Pussiant Magic Theory and Affinity with Magic Theory. I don't say that to discourage you from the journey you want your character to take, only to emphasize that what you think you're gaining from it may not be as much as you'd like.
Thanks for the analysis. You've clarified some nebulous concerns I've been having as I work on Antoninus.
I was looking at Philosophic Alchemy more for vis extraction than for using more vis per season, mainly because most of my Ars experience is from a saga with moderate-to-low vis sources. It looks like Insula Canaria's vis stores and sources are probably large enough to eliminate the vis extraction advantages of Philosophic Alchemy, which means using extra vis per season is the only advantage to gaining multiple mystery virtues. I don't see the value in it, since the items I planned to make 'in bulk' wouldn't be invested devices anyway.
As for Vulgar Alchemy, I'm realizing that I was fixated on finding creative ways to do things with enchanted items and assumed my House would appreciate them. Now that I'm actually trying to think of those creative ways, it looks like I'm building different mousetraps rather than better ones. Yes, finding ways to use Muto rather than other techniques to generate similar effects is neat for Antoninus, but the people I was hoping to impress (House Mercere and the enchantment market) aren't likely to care about that. They just want the biggest bang for their buck, not fancy ways of doing what could already be done. For this reason and the others you pointed out, I probably don't need Vulgar Alchemy.
When it comes to enchantments in general, I don't see a way for Antoninus to fill a niche that a Verditius couldn't, at least without an overhaul of his virtues (and possibly his concept). All he'll be able to do is churn out minor useful items for Redcaps, which may turn a tidy profit for him, but won't gain him any respect. I'm beginning to think he needs a new Phase 1 in his grand plan to rewrite the traditions of House Mercere. I'll get back to you when I think of something better than 'steal the underpants'. 
Final (I hope) version of Antoninus, at saga start. Background is essentially unchanged.
Voting Sigil – cat’s eye made of electrum
Casting Sigil – magical effects appear to ripple outward from the target point
Virtues
The Gift, Magus, Puissant Muto (free)
Mutantum Magic (minor Hermetic)
Tamed Magic (major Hermetic)
Boosted Magic (minor Hermetic)
Inventive Genius (minor Hermetic)
Skilled Parens (minor Hermetic)
Affinity with Muto (minor Hermetic)
Affinity with Magic Theory (minor General)
Well-Traveled (minor General)
Flaws
Deficient Technique [Perdo] (major Hermetic)
Driven (major Personality)
Covenant Upbringing (minor Personality)
Curse of Venus [women] (major Story)
Characteristics:
Int +3
Per +1
Str -1
Sta +2
Prs +1
Com +1
Dex -2
Qik -1
Age: 34 (born 1189)
Size: 0
Confidence: 1(3)
Warping: 1 (6)
Personality: Dedicated to his work +3, Reserved +1, Awkward among mundanes +1
Abilities
Italian (Lombard) 5
Latin (Hermetic) 4
Occitan (Provencal) 3
Gaelic (Scots) 3
French (Norman) 1
Area Lore: Roman tribunal (covenants) 3
Area Lore: Loch Leglean tribunal (covenants) 2
Area Lore: Provencal tribunal (covenants) 1
Area Lore: Stonehenge tribunal (covenants) 1
Folk Ken (magi) 1
Charm (Redcaps) 1
Guile (hiding sexuality) 1
Etiquette (magi) 1
Bargain (enchanted items) 1
Awareness (alertness) 1
Athletics (running) 1
Brawl (dodge) 2
Stealth (sneak) 1
Survival (forest) 2
Artes Liberales (logic) 2
Philosophiae (ritual) 1
Magic Theory (Muto) 6 (15)
Parma Magica (Corpus) 2
Finesse (Corpus) 1
Penetration (Muto) 1
Concentration (spells) 2
Order of Hermes Lore (personalities) 2
Code of Hermes (apprentices) 1
Magic Lore (hereditary magic?) 1
Arts
Creo 8
Intellego 5
Muto* 15+3
Perdo 0
Rego 6
Animal 7
Aquam 2
Auram 0
Corpus 9
Herbam 4
Ignem 1
Imaginem 4
Mentem 1
Terram 3
Vim 9
Spells
Doublet of Impenetrable Silk MuAn 15 Touch/Sun/Ind
Cloak of the Duck’s Feathers ReAq 5 T/S/I
Eyes of the Cat MuCo(An) 5 T/S/I
Aura of Ennobled Presence MuIm 15 T/S/I
The Crystal Dart MuTe(Re) 10 V/M/I
The Unseen Porter ReTe 10 V/C/I
Disguise of the New Visage MuCo 15 T/S/P
Transform the Ravenous Beast to the Torpid Toad MuAn 25 V/S/I
Rise of the Feathery Body ReCo 10 T/C/I
Aegis of Unbreakable Wood MuHe 15 T/S/I (HoH:S)
Coat of a Hundred Colors MuAn 10 T/S/I (Net Wizard’s Grimoire)
Form of the Night Hunter MuCo(An) 20 P/S/I – as Shape of the Woodland Prowler with Personal range, except the caster is turned into a Scottish wildcat
Totals include 8 years of advancement
1 year of lab work:
I've posted this final version of Antoninus to the wiki.
Fabrizio the Cat has been added to Antoninus's wiki page.