Sounds like a great find. Old growth (I assume its old growth) yellow pine is getting very hard to find anymore.
My main concern with putting the wood directly over the vinyl is ventillation. Hardwoods typically have ventillation furroughs cut in their backs. This allows the wood to move a bit and adapt to environmental moisture changes and dry out if exposed to moisture. The vinyl will tend to seal off one side of the wood and if you spill any liquid on the floor that gets through the polyurethane finish, you run the risk of having the vinyl hold the moisture to the wood for long periods. Do you think cutting furroughs on the backs of the pine boards would be advisable?
Personally, I would not use a great find like this on a kitchen floor, where it will get lots of foot traffic and be subject to spills of all kinds. I would use this wood in a living room or study where its age and beauty could be appreciated for many years. This is just my personal bias, but I install kitchen floors with the idea that they have a relatively limited life and will need replacing every 10 years or less. If a wood look is what you prefer, Pergo (or equivalent) is great for this, as it is tough, easily installed, looks reasonably good, is reasonably priced and can easily be replaced without a sense of loss :-)
Just my view, and again, congrats on your find. Where ever you put it, I'm sure it will look great!