I'd suggest you go with the Bryston 3BST or 3BSST for the 2's now and worry about amplifying the 5's when you get them.
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The price difference for a used 3B-SST vs. a used 4B-SST is not large, and my long-term experience with Vandy speakers (any model) is that they perform better with more power rather than less. Therefore, I'd recommend the 4B-SST.
I am using a 4B-ST with my Vandy 3A Sig's, and intend to use it (or a 4B-SST) when I upgrade in the near future to the Vandy 5A's.
If it is a question of "when" you get the bigger Vandy's as opposed to "if" you get.... then pick the 4B-SST. If the price between the 3B-SST and 4B-SST is significant to you then consider a late edition 4B-ST instead. If you are uncertain about stepping up to a larger Vandersteen then you will still find the 3B-SST (or 3B-ST for that matter) delightful. If you KNOW you want something most expensive then save a little more and get it now. Fewer upgrades CAN be a good thing financially over the long run.