I got a vintage modified fender mustang. Two of the strings have a buzz that is heard on open & fretted notes.?

This has nothing to do with strumming or any other way you play the guitar although you can sometimes increase buzzing problems if you play with excessive force.

You either have had the action (string height) lowered, or the neck relief (the lengthwise straightness and curvature of the neck) changed, or there are issues with the frets (can also be any combinations of all of this). Since you have open string buzzing also, it may additionally be issues with the nut.

Since it's a new guitar, I would take the guitar back to have it fixed - this is a warranty kind of thing. Do NOT attempt any fixes yourself - apart from raising the action on the bridge (bridge saddles) as a temporary fix. This is likely a quick and simple thing to deal with for someone who knows what they're doing.

Some times a guitar neck - which is made of wood after all - will move when you move it from one location to another. You may need to have the truss rod adjusted slightly (the truss rod sets/adjusts the neck relief). Again, do not try doing this yourself. Leaving it to some well meaning friends is just as bad an idea.

What went wrong is you bought a Fender product, Fender hasn't been able to fix that bridge rattle sense they developed and started producing those pieces of crap, which would also include the Jaguar and Jazzmaster. There are some retrofit upgrades for it but usually cost more than the guitars worth to completely fix the problem

If it's a new guitar take it back to the store and ask them to do a free setup. You are entitled to a properly set up, buzz-free guitar when you buy it new.