Think you are probably going to have to do a best guess and figure it out.
A 330ml can of coca cola is 7syns (diet would be free, but I guess if it doesnt say its diet then it probably isnt.
A standard (25ml) measure of vodka is 2.5 syns, a large (35ml) measure is 4 syns.
For comparison, the basic foods section says a 330ml bottle of alchopops (WKD etc) is 13.5 syns - not saying it would be as high as that, but worth thinking about.