ok, so I know you're supposed to keep your thumb over the neck of the guitar at all times, right? ...is that right? well I have a really small hand so it gets tricky when i slide or use vibrato. and when I remove the thumb to do these I can't seem to get it in the correct position over the neck of the guitar again, so that screws up my playing. any tips or advice?
Actually correct thumb positioning is flat on the back of the neck, directly behind your middle finger. (But can also be used in the position you stated in certain situations mostly for comfort in songs that don't move a whole lot.) You'll be able to develop more speed and articulate better with your thumb on the back of the neck.

as for your question, It sounds like when your thumb is on the neck of the guitar for everything but you're removing it completely to do slides and vibrato. Which leads me to guess you're gripping the neck to tightly.

Try keeping your hand loose and comfortable on the neck, and keep your thumb on the neck at all times, when sliding and doing vibrato you're thumb is providing back pressure on the neck, allowing your other fingers to move freely while still keeping the string in contact with the fret board.
Thanks guys, this made some sense to me. One more question: Is it true your thumb should ALWAYS be on the back of the neck, even when using vibrato or sliding? I have a habit of taking it off. =p
I don't think there's any sort of rule about your thumb ALWAYS needing to be in a certain place. There are exceptions to everything and when it comes down to it, you need to do what's comfortable for you.

However, I personally think that having the thumb on the back of the neck gives you a point of reference to help your fingers hit the right spot on the fretboard, and it also gives you some leverage.

But do whatever allows you to make the sound you want to make.
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Your thumb shouldn't always be above the neck, this is a myth.In fact, the most common position for the thumb is probably flat on the back of the neck if you know what I mean. Of course, you can bring your thumb over the neck if you're say playing an A chord and want to mute the low E. But I wouldn't worry if you can't get the thumb right above the neck. Just use your thumb to the style of the music you're playing.

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In fact, i saw a guy with his thumb UNDERNEATH the nexk and he was ripping the shit outof some solo's it was immense.

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