30 yr old speakers- time to replace drivers?

You might have to go on Audiokarma for vintage gear and repair of said gear.  It seems some here would report not to bother recapping or reworking any thing over a certain age. I guess we should put all our vintage gear into a landfill! 

The NHT VT is a good loudspeaker, but it's not a special loudspeaker.  I have a pair (VT-2, I think) in a bedroom mainly used for watching video and they work well.  I would advise not putting much, if any money into them.    NHT won't help you with any info on the crossovers or drivers.  So unless you're a real DIY kinda' fella it's not worth the money to even figure out what you would need to change. As long as the drivers are in good shape you're pretty much there.  The loudspeaker are what they are.

A true low cost change would be to replace the internal wiring.

@pureclarity sorry, I missed the problem statement in your OP! do you hear distortions, or any kind of sound problems' to fix? If not, why to “fix” anything?

Speaker design (fix) can be very challenging, and needed some assessment, x-over sims, measurements etc, not available to everyone.

The Xovers are totally worn out ,having upgraded Xovers a long time the capacitors dry out for sure after 15 years or less , you loose detail and then uneven output per speakers ,then just stop working.

I agree, I would not put money into a thirty year old speaker. There have been huge improvements in every aspect of design.