It would help if Winchester didn't label their entire line as 'Vanguard 7700'. I did a search and came up with a ton of different single gun lock boxes - including pistol and rifles. Some had removable combination locks, which in that case just need a simple bolt cutter to snip. Can you take a pic of your safe? Many of their products are made by Stack-On and if it's the one I'm guessing you have, there's a plastic badge, (plate) on the front of the safe that you pop off and it has key access. If you don't have your keys, any competent locksmith can pick it, or Stack-On will send you a new set. If it's electronic, you can go through the mounting hole with a long screwdriver and 'feel' for the reset button. Once you depress it, the safe will beep, you enter your new 3-digit code and you're in...All of this advice is useless, as I have no idea which Vanguard 7700 you have...Good luck....
With a 3 digit combo, it should take under 30 minutes to try all possibilities in series. Boring, but there it is. Turn, click, turn, click, turn, click. Might be possible with a thin strip of metal to trip the catch, but you'll probably use up 30 minutes finding or making one.
Another possibility is that the lock just plain broke.
I had TWO of those cases, and at least one combination lock failed mechanically (didn't recognize the combination) on each, I had to destroy the lock using a large screwdriver to get it open. (I wasn't about to waste 30 minutes trying all combinations).
If there is anything good inside use a screwdriver, pry it open and move on with life.