Alcohol will dissolve the varnish. That is what when Ethanol fuel was introduced, many folks had issues with the fuel components getting gummed up.
On a different note, using a shop vac in a gas tank is a terrible idea. You are lucky the vac it didn't blow up. Seriously, gas fumes will linger in a supposedly empty tank for just about forever and fumes are what explodes, not liquid gas. The shop vacs are not ignition protected.
There are some methods to prevent gas fumes from accumulating; pumping a lot of air through the tank is one. Putting dry ice in the tank is another. It will freeze the gasoline vapors. But no shop vac