"can I use my Synchron libraries with VI Pro and/or MIR Pro?"
I don't think so because I think they use different file types.
As for the differences:
Think of Synchron like a prefabricated home where the walls are already sized, constructed and painted. All you have to do is assemble the parts to make a whole.
The VI series, on the other hand, is like a stack of lumber and cans of paint. You decide how big the walls are going to be what color they are whether or not they have windows etc. Whether the roof will be made of tiles or clay or tin or whatever,. Will there be a pool in the backyard?
So Synchron is a more expedient approach in which you can literally "plug and play" but you're limited in things like venue and sample manipulation.
The VI series gives you much more freedom and flexibility and you have the ability to create your own sonic environments (reverb, atmosphere, venue,etc.)
The downside to VI is the steep learning curve. It's like learning how to play an instrument. Subsequently it takes much more skill and patience to use it.
I don't use MIR so I can't comment on all the particulars but basically if you wanted to record your Hollywood blockbuster score in the Perneg Monastary with all the players placed on stage in their respective spots but the monastary wasn't available (because the monks are too busy with their Gregorian chants) then the next best thing would be MIR. Not to mention all the other venues available.