I'm the same size as you, and my vamp top is a small and fits perfectly; however - if I had smaller boobs it'd probably be too big/baggy on me and I guess I'd need the XS? I'm a UK DD cup
I think that you should go with the bust size for the vamp. I think the waist is a tiny bit stretchy. I had to get the small for the top and the waist still has plenty of room in it and my waist is only a little smaller than yours :)
Definitely go with your bust measurement, the cups are kind of molded so if they are too small you get a weird quadroboob thing going, trust me its not a good look lol I had to go with a large, even though my waist is between a small and a medium because the Medium made me look deformed. And then I felt really self conscious in it.
As you can see, not sexy :)
I had the same problem as you. I'm 32C bra but with push up I'm near 33-34 inch. And 26 in the waist.
So I ordered XS in black... and it was too small. I can zip it but I cannot wear a bra and I am really compress. I had to wear a bolero but when I bought the zebra one in Small, I saw the difference! I was more beautiful and flattering;
And now I'm crazy about vamp top but size small ^_^