Vent it with a spare needle to equalize the pressure before you stick your pin with the water in it.

I always reconstituted with a slin pin 1ml at a time. I always made sure to squirt down the side of the vial and then gently swirl to get it all into solution then I add the rest of the bac water. Whether or not this is sound advice I'm not sure, that's just how I was taught and it's worked for me.