Hello,
I don't believe there is anyway you can manipulate this since when encapsulation happens, physical slices are converted to volumes. So actually you can not run vxresize/vxassist commands because it is not capable to increase a physical slice. Moreover need would be to find contigous space. So no option but to create space somewhere else, move /var for timebeing, create new var with bigger space & mount var back at original.
5.0MP3 introduced some new ways to resize the root /var partitions but as you mentioned you are running old version of veritas, you are out of choice.
Gaurav