vec_set copies (sets) the second vector to the first, it is not a set of vectors. So...

vec_set(screen_size,vector(1600,900,0));

...changes the screen size to 1600 x 900.

I definitely recommend using the manual, and getting used to searching for things in there. But there is also AUM magazine which has a lot of working examples. You can see it on the left side of the main forum page. Note: A lot of the examples are from the older C-Script and will not work in Lite-C. With a bit of understanding they can usually be updated to Lite-C though.