Ok so back to the original subject, for Vegeta.
Heres what it looks like in the show;
So it's basically the same as Trunk's but without the preceding hand gestures. Point blank range, really wide energy blast resembling a flame (there, now everyone's happy) Leaves crispy remains.
How I think it should work;
-Advanced Melee-
Vegeta's doing a combo, if he ends with a fierce hit, he knocks them away like it is now. If he ends with two fierce hits (any direction) Then he hits them with the first and while they're recoiling he quickly extends his arms (one arm for Trunks) and fires the blast. However, it's a death move, which means it'll only fire if the computer knows it'll do enough damage to kill, otherwise it acts as a normal fierce hit.
How it all works out;
If the combo ends with a fierce hit, knocks the enemy away.
If the combo ends with two fierce hits but the attacker has insufficient ki, it knocks the enemy away.
If the combo ends with the victim still having at least 21% health left, the last fierce hit knocks the enemy away.
If the combo ends with two fierce hits, the attacker has over 1/3rd ki remaining and the victim has 20% health or less, the second fierce hit is replaced by the energy flame, that gibs them instantly.
-Stand-alone Move-
Charge is 1 second, range is roughly 1/4 the distance of Buu's Candy attack (around 32 units?), drains ki at the 1.5x the speed as Final Flash, but drains HP at 10HP per second. Instant kill (gib) at 20% or less.
This is all for Vegeta, but roughly the same applies to Trunks. C&C.