I once faced the same problem.
I think it's because vslider = x, y, len, bmap, min, max, var; is kind of an internal engine structure, where the compiler expects an initialization with numeric values before runtime.
The same pbl occurs when you try to do this with TEXT pos_x etc.
While for text the solution was simple (write a function and change these values after initialisation: mytxt.pos_x = 123;, for the slider it did not work for my purpose.
My workaround consisted in builing my own slider with a bitmap, that could be shifted up and down with the mouse. By knowing the actual y-position of your slider-bitmap, you are able to calculate the rest (let lines appear and disappear on top and bottom or vice versa. But it was a pain in the ass to programm all that alone.