It's important to understand that varnish doesn't "dry", it "cures" by combining with oxygen in the air. Once that process starts, such as in the remains in your varnish can, it cannot be reversed and thinning the varnish will not make it usable. That's why it's important to store varnish in small containers and prevent air exposure as much as possible by purging the air from the container with a product like "Bloxygen" (I've heard that CO2 or propane/butane will work if you have them handy, but I haven't tried them myself.)