Well, it is well known, that in Vienna gambit accepted some typical kings gambit ideas work great for white rather then fall short of their target. as in KG. Muzio Gambit should be winning, same for algaier gambit.
I don't really know what is black's best course in vienna gambit accepted ( black schouldn't have accepted first palce !), but it definettaly isn't 5. ...g5. He schould not continue in spirit of kings gambit, as the extra developed knight, and white's control of f6 make HUGE difference.
I just faced an interesting response to the Vienna Gambit accepted which I wasn't expecting, and which nearly (and should have) cost me the game on move 6:
1. e4 e5 2. Nc3 Nf6 3. f4 exf4 4. e5 Ng8 5. Nf3 g5 6. Bc4 g4
According to the engine, 6 ... g4 attacking the knight is black's best try, but I'd never faced this before.
Here, white is +1.27, but only if white realises that defending the knight is unnecessary. The best move is to ignore the attack and push d4 or simply castle. If black takes, white has a +1.8 advantage and can quickly scoop up black's pawns and have a huge lead in development.
I tried to save the knight, and this is losing for white (-2.57 !) (I won my game in the end, but I really shouldn't have).
Fear not the pawn on g4!