MacGregorSailors.com

Where can I find vendor for side lifelines install, DIY, for my Venture 21?

You could check http://shop.bwyachts.com/category-s/109.htm. They don't specifically list them, but might be able to help you.

West Marine has the hardware necessary to DIY, and they have a crimper you can use in the store.

Any rigger at a sailing yard could probably build you a set, if they had the time and needed the work.

Buy the crimping tool. It's cool to do it. And you can do repairs anytime and anywhere.
And even more you can build it maybe even better than stock etc.

I just bought a '85 Mac25 and it's in need of all new lifelines, (they're frayed and grab your fingers).

I bought 2x 25' lengths of 1/4" amsteel and 4 stainless steel turnbuckles. I'll be replacing my lifelines with this dyneema as it's stronger and impervious to rust/corrosion. All I'll have to worry about is chafing, which I can easily check and replace them again when needed in about an hour with some more dyneema.

NOTE: you'll need a fid kit and/or a splicing needle to create the splices, but it's easy to do, and you'll find a million uses for dyneema and the different splices you can do. (soft shackles being a major one - you'll want to make a few and keep them on the boat in different sizes) Also made my daughter a 'bracelet' that is really just a soft shackle made from the smallest diameter dyneema i had on hand, because she thought they looked so cool, and her friends have asked her where they can get the same 'bracelet'.... maybe I should start an etsy store haha!

Hope this helps!