The Problem with --VM (and Poison Magic)
It's that the book has no flexibility (has no utility spells to help you deal with other threats) and that poison resistant things are far too common to rely on the book for long. Several high-tier lair threats are immune, like Spiny Frogs, Hydrae and Black Mambas. And there's a 50/50 chance of it being useless for each lair branch (virtually no use in Snake and Swamp). You may even run into packs of wights before lair that are immune.
No other spellschool has to put up with so much immunity so early and the starting book (and for the matter the spellschool) has no flexibility to do anything to help with these encounters.
I can also say that the start book is very unilateral; offering less variety than --IE, --AE or --EE. --FE is similar, but, --FE has to deal with far fewer immunities in early game (or for that matter the whole game).
Also, now that cure poison has been removed; --VM is the only book start without at least 2 spells of of level 1 and 2. (All others except necromancer have a level 2; Necromancer has two level 1's). Also, since Meph is a 3 school spell, there's a significant early game experience gap between Sting and Meph Cloud.
Before Orc was made so much harder than Lair; --VM could make up for some of its weaknesses by clearing orc first, since Wargs are the only rPois threat there; with the harder orc this is no longer the case (as easily).
I'm not making any specific suggestions at this time just stating facts. Poison and Fire are both similarly unilateral (no real utility spells all damage dealing); but fire is much more viable generally.
In short, Meph, OTR and Venom Bolt are great spells; but when none of them deal with a black mamba at all and you die in Lair due to rPois monsters; it makes you question ever investing in the school at all.
Note: I did personally win FeVM on the first try; but that character both had felid speed to help with things and had a lucky book acquirement book of fire, which allowed much more versatility out it's existing poison spells with Ignite Poison and allowed to branch into fire when resistant monsters were starting to become common. It otherwise would have likely died trying to avoid all the immunes or switching into melee.