There is no "legal" way to buy the Voinian dreadnaught. The only way to get it without cheating or using a plug-in is to get the UE mission to go and scout out the dreadnaught (the one right after you get the cloak), and then when you're in the system with the dreadnaught, abort the mission, disable it, and try to capture it. Of course, it's got over 2000 crew, so you'll need a ship with tons of crew (preferably a UE Cruiser or even better, a Voinian Cruiser), and as many high-crew ships as escorts as you can get. If you don't capture it, then go back and get the mission again, and try it again, until you either succeed or give up in boredom.
Alternatively, you could use a plug-in that lets you get the dreadnaught after completing the Voinian missions. I made a small plug-in like that a while back (heck, it must have been at least two years...). I never bothered to upload it to the internet, though. I'm sure others out there have made similar plugs and uploaded them to the plug-in archives. If not, I can see if I've still got my old "Get the Dreadnaught" plug-in lying around somewhere (it definately wasn't the best work I've ever done... it was my first attempt at making a plug-in ever...)
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A hatch opened up and the aliens said,
"We're sorry to learn that you soon will be dead,
But though you may find this slightly macabre,
We prefer your extinction to the loss of our job."