It's complaining about scalars because when you do vector=scalar, Mathcad returns 0 (because a vector isn't a scalar!). You need to use the vectorize operator to compare the elements of a vector with a scalar ... and then you need some method of using the resultant vector of true and false (1 and 0).
It's probably simpler to make your expression into a function and call the function with the vectorize operator, or put your expression inside a for loop (and use it in a direct program or inside a function).
Stuart
