Can i purge auto ac system instead of pulling vacuum before charging with 134a.?

To accomplish the same thing, no. Pulling a vacuum is how you perform your leak test on the system.. I suppose purging the system could expose a very large leak which probably would have already been detected prior to purging the system, but it's not going to work so well for the more subtle leaks which you may not hear or visually see if you try purging the system (such as a leak in the evaporator). With a vacuum test, the presence of leaks is determined by the ability (or lack of) the system to hold that vacuum. If it can't hold the vacuum, then you know you have a leak, and that's where you do the dye test or the halogen sniffer.

Then there's the question of what exactly you intend to try purging the system with. Air is a no-go, unless you're in the mood to have to replace your accumulator or receiver-drier, whereas that wouldn't have to have been done if the system turns out not to have any leaks.