Well, to further the Vedette example, that 10 tons could be spent in one of 3 fairly simple ways to maintain the AC theme of the machine:
1) A 2nd AC/5 + 1 Ton of ammo, with the remaining tonnage being spent on upgrading the turret ring
2) Trade the existing AC/5 for a AC/10 with 3 tons of ammo, and after again making the turret bigger to fit it leaves you with 3.5 tons of extra armour if my math is right**
or 3) Just add 10 tons of armour. Giving it 16 tons... what a brick
Now options 1 or 3 are probably easiest, and ease the logistical problems if you happen to have a mix of the ICE and FCE versions in the force. Personally I lean towards option 1, maintains the range and ammo load per gun while doubling the firepower. The 8 AC/5's that would come out of a lance of these things is definitely not something to be ignored. Just giving it 16 tons of armour would probably mean people immobilise em and then stay away or rain arty on them.
As for price, that is a bit of an issue, the engine alone adds 380,000 to a vee that is under 750,000 c-bills normally, all up the new twin-AC5 vedette costs 1,037,250 or about a 40% increase. Still 100K cheaper then a standard Bulldog, and on an open field I think I'd prefer 2 AC/5's over a large laser and SRM's, specially as the vedette is a little faster at 5/8.
Tech rating as per Tech Manual is D, just like Fusion engines, but the Availability is a little better the Fusion during the Succession wars, D rather then E. Doesn't match Combustion engines of course, but then no engine is easier to find then an ICE.
**Due to Solaris Skunk Works having issues on my PC for some reason and Heavy Metal Vee not having rules for FCE apparently, I am largely doing this by hand
EDIT: got the availabilities wrong, not sure how when I was staring right at it...