Hi,

Well, this is a young plant so it can be a bit harder to tell one from the other with the usual markers, but either way you don't want to have either type growing there over your shrub. You should be able to pull something this small  by hand but otherwise dig out the crown so nothing remains to resprout. Both grapes and porcelain berry have an annoying habit of continually regrowing otherwise. 

If you break/cut open a stem, p-berry would look solid white in the pith. A grapevine is going to be brown and spongy.  P-berries are smooth, hairless vines whereas most --but not all !  grapes have some degree of small hairs growing somewhere on the leaf underside.  It's too young in these photos to have mature bark, but grapes are shaggy barked and you can peel the bark away while p-berries are simply rough feeling. There's no fruit or flowers to make additional comparisons. 

So, do the stem snap test and see!