I'm having a bit of difficulty seeing how these two flaws are different. From the discriptions It sees like both just end all magical ability when the uncommon circumstance comes into play.
I'm wondering which one would cause more trouble for a magus who's uncommon circumstance is in the presence of adders.
Vulnerable is very different. Cut and pasted from P151 definitive edition for Vulnerable magic.
"This condition immediately ends the duration of a spell when it is applied to the target, or all of your active spells when applied to you."
By my read, any instant spell still works. Vulnerable would be terrible for anyone planning to use long term spells or make items, as with vulnerable the effect is dispelled, while restriction the effect cannot be used, but will kick in again, the moment the restriction is gone.
Anyone not planning on making many items, or cast long term spells, vulnerable would probably hamper them less often. Someone with Vulnerable could arc of fiery ribbons the vipers, but a coat of flames would fail.
The one with restriction, the item/spell that makes them immune to poison; the other one that adds soak so the adders little fangs do much, they are not dispelled, while the vulnerable one is.
I would not allow this restriction, as it is too specific. But I would allow it if we said “in the presence of serpents”. Then it would trigger when any snake is present, but also many dragons, and I could even throw in a banner with a snake on it once just to make the pc sweat.
Vulnerable
"The character’s magic is automatically dispelled in certain uncommon circumstances....This condition immediately ends the duration of a spell when it is applied to the target, or all of your active spells when applied to you." DE p.151
Restriction
"You cannot cast spells at all under certain uncommon conditions. These might refer to your state, such as touching the earth directly or having no beard, or to the target, such as birds or glass, or to your location when you use the magic, such as on a small boat or in a storm. The Restriction also applies to effects generated by any enchanted items you create." DE p.143
So Vulnerable = When you cast an effect on a target and the target meets the condition, the spell is dispelled. If the caster meets the condition, all his active spells are dispelled. Not Magical items, not longevity potions, not familiar bond but It includes Parma, ritual, formulaic & spont spells. This uncommon trigger can be touching iron so the character can mitigate the times it happens.
Restriction = Prevents you from casting also when meeting an uncommon condition. It could be a class of things that are basically immune to your magic which is cool as it doesn't require a lot of effort to make it come to play. If it is a state, then you need to work to always describe things which often lead to the flaws to be overlooked but something like a state of mind could be cool like when angry or bored.
Both flaws can be easily avoided or over applied depending on the troupe. Both can be horrible or fun to play. Just chose wisely.
W
Lets just used a condition of touched by X where X is some uncommon material-
If you have a dancing illusion with a month duration and vulnerability then if you are touched by X the illusion goes away
If you have a dancing illusion and a restriction then you can't cast any spells while you are touching X.
I'm admittedly unclear whether you can cast momentary duration spells with vulnerable while wearing X- are they dispelled the next moment or are they dispelled as you cast them...