Just switching the laser on at the beginning and off at the end of you vector cuts should do the job. That's what I'm currently doing on my 40W laser. The laser head is likely to be very light compared to a CNC mill so acceleration should be high and you'd have minimal ramp up. I'd try this first before going for a more complicated solution.
If you do want to make sure that the laser power doesn't vary with speed then you probably want to go with a fixed number of pulses per inch (or mm). This is an interesting discussion about this. I basically involves counting the pulses Mach3 outputs to the stepper driver. There's some trigonometry involved for diagonals/curves but essentially it's "fire the laser for a fixed time every N" steps.
http://www.buildlog.net/blog/2011/12/getting-more-power-and-cutting-accuracy-out-of-your-home-built-laser-system/