There's a few things here:
- Alcohol is a general preference, not a requirement
The only main requirement for the solution that contains the metal is that it won't chemically react at all or otherwise alter the metal. Admittedly, alcohol is fairly easy to get and consume, for flavor reasons if nothing else. Spook when he's on constant burn in Hero of Ages just uses a mug of water and mixes tin dust straight into it. Later in the series you will see someone use fish oil.
Most vials do not contain more liquid than your average shot glass would. Only the hardest liquors would get someone drunk in that quantity, and only if they were downed with frequency. Vin seems to carry about 3 vials; that might be enough for a decent buzz, but isn't likely to make her fall over. (other answers have covered this pretty well while I'm typing.)
- Higher metabolism burns off alcohol quicker
Alcohol gets processed through the body faster by physically fit individuals with a high metabolism, giving them less time with it's toxic effects. Anyone burning pewter has also automagically juiced up their body, further reducing any drunken side effects. Plus, naturally, our heroes are some of the most active people on the planet. They're gonna burn it off.