what do you mean by merge tracks? you can have multiple items on a layer, or multiple layers for the same speaker if thats what you mean.
I'm not sure if there is a limit to the number of audio layers in Vegas. Its essentially unlimited.
You can set the pan/speaker of an audio layer. If you are working in 2.0 Surround, there is a pan slider on the Audio Layer name. If you are working in 5.1, you can set which speaker, and how much it is isolated on that speaker.