Yes, ilmcuts, you were absolutely right. Thank you for the help. I read the Vtray's readme thoroughly and what I was missing was:
"VTrayCheckOnCommands True"

I thought it much easier to auto-resize the taskbar with the exported variables, like this:
VTrayOnAdd !TaskBarSize $ResolutionX-VTrayCurrentWidth$

I don't know if that's an elegant solution, though. I confess I couldn't get the !taskbargrow/shrink to resize with the proper width (I couldn't calculate the correct number).

Thank you, man.

Edit: are these features also available in Systray2?