+ 1 for Pat.
Alternative:
You never know what medium is on there. Could be high quality stock... could be $hit.
It also depends on the area (location), time of year, heat, humidity etc.
I would:
1) Lightly take a razor blade and cut through vinyl, only JUST touching glass. Do this far left and far right. Start with say a 1-2 inch strip. Increase with the ease - up to whatever strip is manageable - this might be 6 inches.
2) On the left - lift with razor, fold back onto itself and peel down. If good stock, will peel - with glue and leave glass clean as.
3) On the right - heat with low heat heat gun and do the same - lift with razor, fold back onto itself and peel down.
4) Both will give you an idea of what is going to work. One way will leave glue, the other wont. Good vinyls generally hold their glue - cheap leaves the glue behind.
5) Once determined what is working - as in removing vinyl AND glue, soften any remaining glue with turps. Soak a rag, and "whisk" in patches, soaking and leave on for at least 2 minutes. Scrape glue off and wipe onto a terry cloth. Once again soak, and once again scrape. Once this is done, soak a clean terry, and wipe excess, absorbing into the terry. By now, should by extremely streaky. Spray pre wash laundry spray and leave for a few minutes. Clean with a clean cloth. You are now ready to do final clean with a normal glass cleaner. If still streaky - add some white / metylated spirits to the glass cleaner to add some "oomph".
6) Remember: let the solvents do the work - not you. Human efforts with NEVER dissolve glue - you have to break it down chemically and structurally step by step. Time is all that is needed, and if done right - will work in your favour and actually save time in the long run.
Cheers- G