Finally I get snapping with interact. Here is the small snippet to do that, simple isn’t it? I’ve been fighting with this two weeks, and really I’m not sure what is different with my last tries.

UPDATE:  Finally I found what is different (the relativePoints with .5), I really don’t understand why, but if I try to use relativePoints to 0 or 0.0 or .0 it does snap but not in the position I wish. I’ll try to understand why this happen but reading the star example of interact they also use it, and I also don’t understand why.