I suppose it could decide on how you're cycling your tank... but if you're simply adding household ammonia then there shouldn't really be any need to gravel vac.
Terry.
i'm "feeding" my tank with fish flakes. I'm worried that if i vaccum my gravel and do a water change might disrupt my cycle and kill good bacteria.
It may well do, but then it could also release alot of Ammonia in one go, bad news if you have fish in your tank.
But as you don't have any fish, play the patience game. Like you, i started a fishless cycle with fish food (to this day i believed it helped in some way) but then followed advice on here and used the 'household Ammonia' way.
There is the option of adding fish and allowing them to cycle your filter, but that method is not recommeded on this (and other) forum/s.. you put your fish through a terrible amount of discomfort, potential disease and usually death, by the time you have finsihed. Spend the money on a bottle of Ammonia, or better still get someone to 'donate' some of their filter media to you (sponge is perfect enough).
Best of luck.
Terry.