If the vents are fairly small, I'd be inclined to put 4 in there - one in each corner on the back. The better the ventilation the easier it will be to maintain a gradient in warm weather.
It's easier to fit extra vents now rather than later, and can always block them off if the humidity is too low by taping a piece of card (or something) over them from outside.
If they're large vents I'd go with the warm end one at the bottom and cool end one at the top - that way warm air will travel down the viv and you'll know it is being heated properly.
If you have a large vent at the warm end at the top a lot of the heat will go straight out and cold air will be drawn across the viv from the other end.
It's easier to fit extra vents now rather than later, and can always block them off if the humidity is too low by taping a piece of card (or something) over them from outside.
If they're large vents I'd go with the warm end one at the bottom and cool end one at the top - that way warm air will travel down the viv and you'll know it is being heated properly.
If you have a large vent at the warm end at the top a lot of the heat will go straight out and cold air will be drawn across the viv from the other end.
