So you make spell that takes someones blood out at a steady rate. You can do this with a level 10ish ReAq spell from an open wound for instance. Would this cause fatigue damage? Deprivation rolls? Either seems plausible as blood loss leads to exhaustion, to unconsciousness and then death. Asking for a friend who is totally not a vampire.
I think you may be underestimating the level of your spell. Remember the unwritten rule of spell design: if the level seems unusually low compared to other spells with the same lethality, that’s because it is.
Let’s call moving blood out of a human body ReAq 4, “forceful but calm.” You’re pulling it out of a human body, so you need a Corpus requisite. You’re also going to be using Target: Part. So you’re at ReAq 10 for a momentary touch spell. So we’re in the neighborhood, but you probably want it usable at range and with a longer duration.
As to the effect of your spell, see The Wound That Weeps, which is almost exactly the effect you describe. It’s specifically a very bloody wound. Inflicts a Light Wound, with no fatigue or ongoing effects (beyond the -1 wound penalty, of course).
If I were your vampire, I’d just learn Wound That Weeps, unless you’re an Aquam specialist with MMF: Blood, in which case I’d design the spell effect I outlined in the first paragraph, inflicting a Light Wound, and make it a Concentration effect, inflicting a Light Wound every round.
My issue with this interpretation is that blood loss would eventually kill. It's why I think either fatigue damage or deprivation is more appropriate. Plus recovering from blood loss is much faster and requires far less medical care than healing a light wound. The question is straight fatigue damage vs deprivation rolls.
I don't always subscribe to the "design should have the same lethality". For example torrent of water does +10 for 4th magnitude, where Crystal Dart does +10 for 2 magnitudes. Incantation of Lightning and Ball of Abysmal Flames are both 7th, but lighting knocks you off your feet. Sometimes the way you go about something is more effective or more difficult one way than another.
In this case, taking blood from an already exposed wound would be easier with ReAq than just taking it from your body with a level 25-30 ReCo effect "direct the flow of bodily energy" or "Transport a target instantly x distance" is the closest I can come to move your blood to a jar.
To further add to this Phlebotomy (Blood letting) in A&A p.60-61 only causes fatigue wounds. The vampire thing was just a bit of humor
PeCo 10 - Cause the loss of a Fatigue level.
I was using that as an example so, let me clarify. I'm looking at how you treat blood loss as a whole. Whether it is draining someone with a spell or bleeding them out on the battlefield. Now looking at the rules I'm inclined to say it's similar to deprivation, with the loss of long term fatigue till you die. Plus recovering from blood loss seems to fit 1 fatigue level per day. Even though it seems I answered my own question , I was wondering if anyone else had address this in their game.
But looking at my example effect: For guidance purposes, as I understand it, blood outside the body is Aquam. Blood inside the body is Corpus, moving blood inside the body somewhere else would be ReCo, as per A&A guidelines for magical surgery. Your effectively bloodletting. If you wanted to destroy blood inside someone I would go with PeCo. So moving blood, not destroying blood. If the form has guidelines that fit, I use the guidelines. If it doesn't then I look to other forms. From an open wound "ReAq Level 4: Control a liquid in a forceful but calm way, such as a fast but constant current." fits without the need to change it or balance it IMO. An open wound is already bleeding, your just helping it along.
I don't see a need to balance this to much as I can make a level 15 or 20 PeTe spell that will kill people and a well placed level 25 I can take out a whole fighting group. Killing people with magic isn't very hard, you just need to be a little creative.
Frankly, Ars Magica doesn't really distinguish much between different sources of damage - broken bones, tissue damage, blood loss etc.
As such, a functional approach tends to be best - what purpose will this spell have in your campaign? If it's just to kill people, my advice then is to just use a normal 'Inflict X Wound' guideline and flavour it as resulting from blood-loss.
Alternately, maybe crib from the Heat & Corrosion section on page 181 for some ideas on how to represent Damage / Time in game terms...
Consider the Deprivation rules, p 180; you may be able to adapt them to blood loss.
There may be more particular rules in Art & Academe. I don't recall any, but it's been a while since I looked and there is an entire chapter on medicine and disease.
For the effects of that sort of thing I always hand-waved long term fatigue loss as a method of getting people to know things are happening.
As far as how to do such things with a spell, if the blood is still in someone's body (or in the wound, which is part of their body) then I'll say it's a corpus effecting spell, rather than just Aq - human bodies are a whole, and blood is part of that whole. Kind of like how I won't let someone ReCo teleport someone's head a foot to the left with a low level part-target teleport - that there's a kill spell, base PeCo 25.