Three easy ways, (assuming full version of Vegas, 6,7,

1 grab the end of the clip while holding the CTRL key and the hand should change to a wavy line then pull or squeeze as required.

2 Right click on clip and select Envelopes>Velocity, then use as you would rubber band audio

3 Right clip on clip and select properties and there is a an option at the bottom of the box for speed/Velocity

You can use 1 and option 3 together to get a faster speed.
once a clip has it's speed change a zig-zag line runs through the clip on the time line

I sometimes find easier to split clips and then do the speed changes rather then use option 2

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