Hey Dawg,
Glad to see you over here at AC! It's a nice place to hang out too. Just wish that Brian were still here to help you out. He'd have had all the parts that you needed. But, John is a great guy, and won't steer you wrong!
Like you, I have a couple of VMPS woofers, where the phase plug popped off. To me, it looks like Brian originally used hot melt glue, to adhere the plugs. Good, but I think it dries out over time, and it's not the strongest adhesive either. Maybe try a multipurpose epoxy. Just be careful not to use too much, and get it into the voice-coil. Shims are your best friend!
As John stated, Parts Express is a good source for L-Pads; Madisound too:
http://www.parts-express.com/cat/speaker-l-pads/306https://www.madisoundspeakerstore.com/l-pads-attenuators/A 15 watt for the tweeter, and a 50 watt for the mids, should be fine.
As to the age of your speakers, Stereophile reviewed the earlier SuperTower IIa/R, in 1986. So, since your cabinets have the different driver layout, and rounded cabinet corners, those were probably introduced mid 90s.
http://www.stereophile.com/floorloudspeakers/486vmps/index.htmlThis might help some too:
http://pointillistic.com/vmps-audio/aboutVMPS.htmPlus, the same woofers, even the Goldwoods, in a stand alone Larger sub:

Finally, the Masters, Brian and James!

Good Luck!
Stimpy
Oh yea, the speakers look great. Nice 'scroe'.