BAT VK-3iX vs ARC LS-15

I am thinking about adding vacuum preamp for my system, which consist of Threshold SL-10 Pre, Aragon Palladium monoblock amps, and Magnepan 3.5. Would changing to either of these vacuum preamp help?
You don't say what needs help. If I had to guess, given the components you list, I'd say the problem area is in the high frequencies. No preamp is going to make the aluminium tweeters in your 3.5's sound like a soft dome and no preamp will make your amps sound like 300b's. You may be able to "soften" up the sound a little, but the underlying problems will still remain. Buying components with the intention of fixing the sound of other components almost never works. Every piece in the system has to be able to stand in its own, without any help. So, to answer your question, the answer is no. You may be able to point yourself in the right direction, but it wouldn't be a big enough change.

However, the preamp is the heart of the system. To get any system right, you have to get the preamp right. If you are looking for a component to build a system on, then that's a different issue altogether. Of the 2 you list, I like the BAT for both build quality and sound quality. (I'm not saying the ARC isn't good, its just not my preference.) Going with the BAT will give you a good place to start building a system you like. It just won't be enough to finish the system. You'll need to do more work.