Vinegar should work on SLIGHT rust. Problem is the rust will return if fuel level is kept low for any amount of time. Especially in humid climate. Tank should be coated after cleaning with appropriate coating. Might be cheaper to buy a new tank.
phosphoric acid aka rust converter if you can find the straight stuff. normal fero-pro contains some sort of plasticiser too. could contaminate. might not
having access to a rumbler and small 5mm ceramic beads helped in my case, but they arent exactly common... but the tank in question was pretty bad, came up like new
if its only minor...dont worry about it. the oil in the fuel should prevent it spreading, and...dont use ethanol fuel...