If I have more than one VPS, how is traffic counted against my bandwidth quota?

Bandwidth from VPS' belonging to the same account is "pooled."

That is, if you have 1 VPS with a 400GB quota and 1 VPS with a 200GB quota, you get 600GB in total to use in whatever distribution you like.

1 VPS can use all 600GB and the other can use 0GB, or vice versa, or any combination in between; as long as the total transfer does not exceed 600GB, you'll be under your bandwidth quota.