Slave capturing? I'm not really sure if that should be in spoil tags or not... then again who has not beaten Fallout 3 lol. But really, every time i attempt to capture a slave, either their head blows up or they just attack me when i zap them with my mezmorizer... so would speech or something increase the potinency of it?

No. The Mesmetron just simply has different affects on different people. For Raiders, Enclave, and Talon Company...i'm pretty sure it will always work. But for the unique characters...it's different for everyone. Here is the link to the Fallout 3 Characters; http://fallout.wikia.com/wiki/Fallout_3_characters Each character with a Jail cell sign next to their name is able to be enslaved, via Mesmetron...without any difficulties or problems. Ones that do not have the Jail Cell sign next to their name, will either get their head blown up or turn hostile, upon Mezzing them.

Using vats can help a lot.

If you get your Energy Weapons skill high enough that will happen less.