Vulcanizing Compound for Tire Plug?

I recently noticed a nail in the outermost tread on one of my tires. As it's in a location a tire shop probably won't want to touch, I plugged it myself. It's been holding air just fine now for a couple weeks and several hundred miles, but I check it regularly and it appear the plug is very slowly working its way back out. I've cut it flush now several times, and after a week or so there is a little more sticking out that I can trim off. The plug repair kit I used did not come with any vulcanizing compound, so I'm wondering if that might be the issue/solution to keeping the plug firmly in place?

 

Ive had plenty of repairs done by san diego tire shops in that precise area, even worse...
Dont go to a big corp tire shops as they wont touch it, find a garicia's or chueys tire shop, not worth messing with....
they will plug & patch from the inside....
$20 well spent, vs you buying vulcanizer & will never use it again...