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Instead of a vector mask on a pixel mask, another possibility is to use a 2nd pixel based mask. 

I don't do this very often, but when I do, it's more for convenience and speed than absolute necessity.  For example, suppose I have one layer mask / alpha channel that selects on the basis of luminosity and a second one that selects some area or  set of areas in the image, and I want to restrict some adjustment to just the brighter pixels in those areas. 

I could do this by multiplying the two alpha channels and forming a new single layer mask from the result, or I could apply the two pixel-based layer masks sequentially to the same adjustment layer. 

One way to do this is shown in the attached screen shots.  In this (not very artistic) example, suppose I wanted to warm up only the very brightest areas of sky.  If I used just a luminosity mask, the bright wake of the boat would also be warmed up.  If I used a mask that selected just the sky, it would warm up both the dark and light areas of the sky.  However, by using both, I can select only the bright areas of the sky.

The attached screen grabs illustrate the technique.  The file names describe each screen grab.

HTH,

Tom M