Trex is not made of PVC.
The original Trex was a composite of recycled low density polyethylene and sawdust.
The current line of Trex has the same core but now includes a co-extruded "shell".
Trex does not say what the shell is made of. It might be PVC but the core is still ldpe and sawdust.
"In 2009 Trex introduced a ‘Transcend’ line of decking  that is a co-extruded composite decking product that includes a shell around the composite boards on three sides to protect the boards from staining, fading and scratching."
Adhesion test results by West System epoxy;
"Different methods of surface preparation are the variables for this test. With the Trex Decking, any surface preparation (including none at all) results in substrate failure
, an indication the adhesive and adhesion are adequate for the material. Even with no surface preparation, the surface seems to be rough enough to provide a good grip for the epoxy. "
I don't know anything about the new shell but it looks like this stuff needs to be mechanically fastened. The substrate comes apart too easily if a really good adhesive is used. Maybe a flexible adhesive caulk would work.
Here is a high res photo of a cross section of a Trex Transcend plank. It looks like the shell is still made of PE.
If joining two sections of shell then acrylic, urethane, or adhesive caulk should work fairly well.