Drive the car around and see how the cooling feels with the R-134 and go from there.
As far as the Freeze 12; yes, I would vacuum the system before filling.
Check and see if the oils in Freeze 12 are compatible with the ester oil you put in when you were doing the conversion to R-134. If not, flushing needs to be done.
At this point before considering the use of Freeze 12, ask others who have used it. There are a/c forum boards that you can get good info from.
It's alot better than trying this and trying that in order to get the desired results with you a/c system.