You should have put it in (if you use vinegar at all instead of better options), let it warm up and run another 15 minutes, shut off and then an hour later (really I mean just wait till cool enough to depressurize the system) checked the pH. If it is still nearly as acidic you are done. If it is nearly neutral, you need to repeat with fresh vinegar, leaving reacted away solution in is doing no further good. You can get cheap pH test strips on ebay or Amazon. They're not ideal but "good enough" for this purpose once your coolant is replaced with vinegar and water so what you are checking has a low color density.
If it was really crusty inside, odds are that you will need more than one cycle with fresh vinegar, or any other low ~5%-ish acid solution, that I assume you diluted even more to make up the total cooling system capacity, that you had already flushed it and had 1 gallon vinegar plus water topping it off.