Have you ever had a bad experience with flax seed? I tried it once, and it seemed to make the cake very greasy- it leached liquid onto the cake board and looked just terrible, and it had a very wet, dense (not GOOD) flavour.

I will try your vingar/baking soda approach, however, and hope it works out. I do alot of cakes for vegetarians, but vegans are always so disappointed when I tell them I won't do a cake without eggs because I cannot guarantee the results.