Yathers, what part of the world are you in? It's a cinch to buy online battery packs, and 30 mins of life after 12 years is pretty remarkable actually! (Working at all is remarkable!).
One of the most popular US sites is:
www.all-battery.com
For a 6V NiMH or NiCd pack...
http://www.all-battery.com/6vnimhbatterypackseries.aspx
Nixx packs today have much better life than they did at the time of your VistaLights, and you can choose the total life you want on them by buying a properly sized pack. Are you (potentially) running both lights off one pack? If so, then you have a total 25Watt/6V = 4.16Amp draw. So take that number (4.2A, or 4200mA, rounding up), and use that number to calculate your pack runtime.
So for one hour runtime with both lights "on", you'd buy a 4.2amp-hour (Ah) pack (or 4200mAh pack). 2 hours, double that, etc...
A good choice might be this 5-D-cell 10Ah pack...
http://www.all-battery.com/nimh6v10000mahbatterypackwithbareleadscustomize.aspx
which would give you over 2 hours of runtime (batteries, as everything, are overstated a little usually in the realworld).
Use that, cut your current connector off the old battery pack (if cabled), and splice it into the new battery pack, using heatshrink to seal both individual cables, then a larger one to cover the entire connection joint.
I'd also recommend a new charger (a larger capacity battery takes forever to charge on a low-rate charger, and the above pack is NiMH which needs a different profile than NiCd (some charges work with both).
http://www.all-battery.com/smartuniversalchargerfornimhbatterypack24v-72v.aspx
You could also choose NiCd packs on their site, keep your charger, and just be very patient when charging. (Your charger outputs at 300mA... if you get a new 10Ah NiCd pack and it's fully discharged after heavy usage, you are looking at 10000/300 = 33 hours per full charge. For a new charger and NiMH pack (the above charger is listed as having a 1.8A charger option), then it's only 10/1.8= 5.5-6 hours of charging time.
Anyhow, hope that helps, some things to think about at least to start you on the way. That's about $60 in pack and charger options... for $80 you could look at a new Magicshine light, with Li-Ion battery pack, and charger... it's hard to justify keeping 12-year old halogen light/nickel battery technology running given the current market in high-power LED lights, but that's a personal choice of course.
