If you don't know the first thing about the batsman, set a regulation field, 5/4 or 6/3, cover mid off etc. If you see the batsmen doing things such as hitting in a particular area or walking across their stumps, move a fielder either to stop them doing it, or encourage them to do it more.
An example is when Shah came in to bat the other day Australia had no mid-on or midwicket and got Shah walking across his stumps.
Also, act on your instinct. It's a bit like bowling, knowing when to try something different.
I've been a vice-captain before, but I would have loved to have been the captain, so you should definetly do it