hit.x gives the position, where the trace hit and because a position is a vector of 3 variables, it doesn't make much sense to add 1. If you want to change the position, you have to change the components of it. e.g.:
//move the position up by one quant
var vertex_num = ent_nextvertex(you, vector(hit.x, hit.y, hit.z+1));
PS: this function doesn't affect the position of a vertex. it just looks for the vertex that is nearest to the given position
