The Vienna Gambit isn't really a gambit, and the pawn should not be taken. However, most players I've faced <1200 take the pawn and it's usually a pretty easy win from there.
Gambits...which to take and which to decline

#18
Accept them all.
Only exception = Queen's Gambit, though you can accept that as well.
You seem to be ignoring the Blackburne-Shilling Gambit. Accepting it is disastrous.

#18
Accept them all.
Only exception = Queen's Gambit, though you can accept that as well.
Another exception is the Milner-Barry Gambit.

@MyNameIsNotBuddy those gambits you take the first pawn but the second one isn't a gambit... just a line with a trap...

The old adage is "The best way to refute a gambit is to accept it". Although basically true, the problem is that when your opponent surprises you with a gambit, they will undoubtedly have studied it deeply while you won't be sure how to proceed, so look for the most solid way to decline. If you know your opponent likes a certain gambit, scope it out pre-game and accept.
Whether you accept the gambit and suffer through a powerful attack or decline and get an uncomfortable positions, check out the theory thoroughly during your post-game analysis. Next time you'll know what you want to do.
#18
Accept them all.
Only exception = Queen's Gambit, though you can accept that as well.
Why?
Well, sometimes it's bad to accept the queen's gambit

#1
"Is there a "GENERAL" rule about accepting or decline gambits?"
Accept them all. Take the pawn, defend, win the endgame. It is easier said than done, but each time you fail, you learn. Think carefully about how to organise your defence.
Do not get greedy: 1 pawn is enough to win. If your opponent offers a 2nd or 3rd pawn then it is often wise to decline the additional pawns.
you can take all the pawns in the danish gambit

Why play the Schlechter Defense? Why give back the pawns that your opponent just gambited when you can easily win with your material advantage?
Personally, I often play this:

#18
Accept them all.
Only exception = Queen's Gambit, though you can accept that as well.
Why?
Well, sometimes it's bad to accept the queen's gambit
I think Kasparov used to use QGA.

The old adage is "The best way to refute a gambit is to accept it". Although basically true, the problem is that...
The problem is that it isn't true. Some gambits are objectively best met by declining or countergambiting.

The old adage is "The best way to refute a gambit is to accept it". Although basically true, the problem is that...
The problem is that it isn't true. Some gambits are objectively best met by declining or countergambiting.
Emphasis on old adage.

The old adage is "The best way to refute a gambit is to accept it". Although basically true, the problem is that...
The problem is that it isn't true. Some gambits are objectively best met by declining or countergambiting.
like say the vienna gambit

#18
Accept them all.
Only exception = Queen's Gambit, though you can accept that as well.
Why?
Well, sometimes it's bad to accept the queen's gambit
Queen's Gambit Accepted is a totally fine alternative to accepting the Queen's gambit that offers rich, open, and tactical play as opposed to the closed, positional nature of the Queen's gambit Declined, Slav, and Semi-Slav

just a minor addition to what's said, you can take the pawn in King's gambit if you prefer but never take the pawn in the Vienna gambit. They make look similar, but the king's knight blocking your queen makes a substantial difference.

just a minor addition to what's said, you can take the pawn in King's gambit if you prefer but never take the pawn in the Vienna gambit. They make look similar, but the king's knight blocking your queen makes a substantial difference.
also important to differentiate the two f4 gambits in the Vienna (Are they both called the Vienna gambit?), 1. e4 e5 2. nc3 nc6 3. f4 and 1. e4 e5 2. nc3 nf6 3. f4. Very different from each other
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#20
"Why?"
In Queen's Gambit Accepted you cannot hold the pawn, so you have to give it back and you can just as well decline. It is a matter of taste. Queen's Gambit Accepted gives a more open position, Queen's Gambit Declined a more closed position.
All other gambits are better accepted. With the extra pawn you win the endgame if you defend well.