...there isn't a huge difference going into a fight with Drain on cooldown and vim for a couple of spells compared to starting with a full bar and having Drain in reserve to recoup your opening expenditures?
In theory, there could be. There would be if, say, there was a tank Vim class. One with relatively poor damage.
In practice, there's this thing about Vim. It's given to two classes that are hands down the biggest damage dealers in the game...and then you refund your killing spell's Vim, with a Willpower based bonus(and you'll get some incidental Willpower, even if you don't build it, enough to get 3-6 Vim back).
It was sorta a problem when Corrupted Strength didn't refund 8 Vim for kills, but now, you often get Reaver skill Vim back anyways.
Both classes have this tendency to drop random enemies like rocks, though. In Corruptor's case, it specializes in class based critical bonuses that make Drain crits do insane(500+) damage quite quickly(30+), which means a full restore. Even early on, Drain sucking up 30-40 Vim is not hard. Going into the fight with Drain on cooldown doesn't matter when it's less "a few spells" and more a full tank of Vim, though.
In Reaver's case, it requires more effort, but they tend to not use tons of it, either, generally tossing out 10-15 Vim with skills. The most expensive Vim skills they have are Dark Portal(an escape) and Vimsense(which refunds more Vim if you're using it with any Leech at all...and why you wouldn't is beyond me, for Reaver at least.).
In both cases, your opening expenditure is recouped, at least against random enemies, by your spell killing the enemy. Against a boss, you should have kept your Vim high enough before that to not be an issue-If you're playing well, you should only be running low going into a major (durable) threat because you ran into multiple major (durable) threats just beforehand. It happens once in a while, but that's part of the class' fun.
The short version is, you cast Drain first on anything you see, as long as there's no major danger, and you get into that habit-and, since your resource doesn't drain fast, due to the refunds on deaths with a small bonus, you have the Vim to avoid the dangerous potential of using the wrong skill in a really risky situation. Once you learn that, you're pretty good.
It limits your play, somewhat, and that's one of the neater elements of Reaver and Corruptor. But with good play, the actual benefit of not having to use Drain-which is a solid, if not special, spell even outside of its Vim drain-is admittedly existant, but really hard to pin down, definitely minor, and certainly not spending tens of thousands of turns spamming a wand.
And, as a note, I'd rather see the wands removed than Vim get a normal rest-to-full mechanic. It doesn't fit it-the current mechanic of requiring tons of sacrifice to fill it up fits more.
However, here's a thought for formalizing it-make it a representable cost.
The most thematic way I can think of to do this is to add a skill-call it Vim Extraction, say. As a basic skill to both classes, like Spacetime Tuning, only one level. That allows you to convert any powered by nature item into Vim. Say, maybe (Willpower/10) Vim per Gold you would have gotten from the item selling/transmogging.
This makes thematic sense-you're literally sucking the natural life force out of the item. It's certainly only good in emergencies-you won't get full power from it from one item. You can only carry so many powered by nature items around anyways, so there's a strong limitation. And it doesn't really interfere with how they play-you can't possibly eat items constantly without a real cash loss, if at all.
And it still makes for a much less tedious way to produce Vim. It just allows you to pay a cost that isn't tedium.
Thoughts?