I will start by answering the last question first.
It could be a problem because you seem to be using the word "vitality" to mean two (or three) different things.
When you wrote "enhancing the life and vitality of the target" I understood vitality to mean "life-likeness" which poses a problem of "how will the sigil manifest when the magus casts a spell on something that is not naturally alive?". You then specified vitality to mean something like "conformation to the platonic ideal of whatever the target of the spell is an example". Then at last you specify vitality to mean "the thing faeries consume to survive and grow".
I personally dont thing that the "vitality" consumed by faeries bears any relation to a person or objects level of conformation to the platonic ideal of their thing. So already at that point you have a problem.
For me the vitality that faeries consume is a realness that comes from the fact that humans have Souls (as in the Divine soul that can pass to heaven) because this Soul is also the origin of things like creativity and true feeling (and True Feeling), and faeries being essentially fake lack those things but need them to survive. In a sense a faerie needs a role to fill but is completely unable to come up with a role on its own and completely unable to change the role without outside help. It needs humans to come up with a story for it to act out and once it has taken a role in that story it is unable to come up with variations of the role until and unless a human has come up with that variation first. In a sense it is a robot reading out lines from the script with no thought given to how these lines fit into the larger play.
I will not 100% guarantee you that what I just stated about faeries is the party line on faeries but it is at least reasonably close.
This interpretation of the "vitality" consumed by faeries is fundamentally incompatible with the vitality you propose for a sigil. Because whether or not something conforms closely to its platonic ideal does not in any way transfer creativity or Soul to it.
In short your problem is that you use the word vitality to mean different things and just because these different things are referred to by the same word they dont actually have anything in common.
On a lesser note the term "vitality hungry faeries" is an oxymoron kind of like "magical wizards" all faeries are vitality hungry, that is part of the definition of faeries in ArM5. Which means that the flaw in question is really "Proposed Major Flaw - Supernatural nuisance(Faeries)". Maybe it does not matter or maybe the player in question is trying to use linguistic tricks to make the flaw seem bigger than it is. Personally I would have no problem allowing a flaw like that but that is something you would have to consider.
It seems from your OP that you are going to be the storyguide so you will have to weigh this for yourself. Especially the questions of "Am I going to actually introduce faeries to bother the player to the degree that I (you) personally feel justifies a major flaw?" and "Do I care?". You should also talk to the player about what they want out of this flaw, because if you actually play it out it is going to take a lot of effort of the storyguide to come up with good faeries and if the player takes the flaw expecting lots of good faeries and you dont provide them then that sucks. But on the other hand it is also possible that the player took the flaw in an effort to "get off the hook easy" and acquire some virtue points with which to buy cool virtues. There is no way for me to know which but it matters and you should try to find out.
I should say that I dont view it as a bad thing to try to "get off the hook" in regards to flaws, Certain Personality and Story flaws seem to pretty much be in the game for the explicit purpose.