...which fabric is it exactly? And how dark is the grey - is it charcoal? If you have enough of that cloth for a three piece, I'd recommend having a three piece made up in it now, as you can always find a different cloth for a contrasting ("odd" is the traditional term) waistcoat later. For an evening event, I think a dark suit with a matching waistcoat tends to look better, whereas odd waistcoats are more of a daywear or even "country" thing (it works very well with tweed).
I think the best configuration for a dressy evening suit (although as it's a ball, have you not considered a dinner jacket?) would be the following:
Coat:Single breasted, one-button (perhaps a link-front closure), bellied peaked lapels
Double jetted ("besom") hip pockets - as seen on most dinner jackets, and no ticket pocket
Either no vents or double vents - Ventless is more formal, but might be less comfortable. Do not get a centre vent
WaistcoatDouble breasted (6x3 or 8x4, in a "keystone" stance that gets narrower as you go down) and with holes for a watch chain if you want to wear a pocket watch.
Peaked lapels
Flat across the bottom (this is my personal preference, although DB waistcoats with points are not unheard of)
TrousersHigh-waisted, fishtail back (cut for braces)
No turn-ups (cuffs) at the bottom
As for the silhouette that will work best for your build, have a read of
this first (by the author of "The Suit: A Machiavellian Approach to Men's Style") and see what you think.
EDIT: Most tailors will have certain silhouettes that they're best at doing. Decide what you want first, and then find a tailor who specializes in it.