I believe plasma rifles were introduced in TW and TM, so you won't find any prior fluff about them.
Against vees: You take the 10 damage, the 1d6 heat is converted to 2d6 damage and added to the 10, and the whole thing is applied in 5-point groups. So if you hit a tank and roll a 7 on the 2d6 roll, you'd apply 17 damage in groups of 5,5,5, and 2.
I *think* shooting at Protos works the same, but don't quote me on that.