It is unfortunate vector smart objects' anti-aliasing cannot be turned off in Photoshop. A solution would be to create two copies, with the top one the anti-aliased version, and the bottom one the non-anti-aliased version.
The bottom version would remove the seams automatically.
To remove any edge artefacts introduced by the bottom non-antialiased version, a layer mask could be introduced and reduced a bit by applying the minimum filter.
I solved the issue in Photoline this way, because I was experiencing the same issue there:
The solution was quite simple, because in Photoline anti-aliasing for vector objects can be turned off:
Perfect version, with nice anti-aliasing around the edges, and no seams! :-)
Now, if only we could turn off anti-aliasing for smart objects in Photoshop, it could be resolved in CS6 as well.