"Peacevine" seems to be listed on some sites as an heirloom and very well could be. If so, it's probably what's called a "created heirloom". It has been bred and grown out over a a number of seasons to stabilise the genes to produce true-to-type. Not all heirlooms are old varieties. Not by any means. All "heirloom" means in this sense is that it will - or should - grow true-to-type.
ChapelGirl, cross-pollination in toms is largely down to insect life buzzing backwards and forwards between varieties and transferring pollen. Separation is one way of trying to avoid it. A guaranteed method is to "bag" the flowers to stop insect life getting at them.
You can make your own bags easily enough. Tulle is a good material. It lets in plenty of light and air. I buy some, cut it into decenty-sized squares, and fashion them into a bag shape.
The key is to bag the flowers before they open, before they are vulnerable to insect life.
Select a cluster of unopened flowers. Carefully nip off all foliage around and close to the cluster. This is to avoid the foliage growing and filling up the bag. Slip the bag over the cluster and tie it closed just tightly enough to secure it. If foliage develops inside the bag, slip off the bag, remove the foliage and replace the bag.
And, of course, not all flowers produce fruit, so it's a good idea to bag several different clusters to improve your chances.
The bag stays in place until the first sign that the flowers have set fruit, until you see the wee pea-sized baby fruit. Having been protected, you know that fruit is pure. Remove the bag, but tie something like a bit of ribbon to the cluster in order to identify it as pure. It's very very easy to forget which cluster of fruit is pure. I've done it!
If you don't fancy buying tulle and cutting it up, you can always just cut the foot section off a pair of light-coloured pantyhose (or those short stocking things) and use that as the basis of the bag.
You don't have to wait till the fruit is ripe to save the seed. Any time from about 30% ripe onwards will produce viable seed.
Welshonion is right. It's a good idea to ferment the seeds. Apart from eliminating any potential nasties, it makes it much easier to get rid of the gooey seed gel.
Cut up the tom and squeeze the seeds and as much of the tomato's juice as possible into a glass jar. If there's not a lot of juice, add a tbsp or so of water to make sure the seeds are well covered by liquid.
Put the jar in a warm spot but not in direct sunlight. Depending on the temperature, you should see a mouldy crust forming within a couple of days. The concoction will also start to stink a bit. They're ready for the next step - washing and drying.
Half fill the jar with fresh water, swirl it around vigorously, then let it settle. You'll see the seeds gradually sink to the bottom of the jar with the gunky mould and seed gel still in suspension. Carefully drain most of the excess water, tipping out the gunk, without also tipping out the seeds. You can either repeat the process two or three times until the water is clear, or, after most of the gunk has gone, tip the seeds into a sieve and blast them under the tap.
Then you dry them. Don't try drying the seeds on tissue paper or kitchen paper. The seeds will stick to both and you'll never get them off. Coffee filter paper is the go. Spread the seeds out on the coffee filter paper and leave them somewhere out of the way of direct sunlight until they're perfectly dry.