Hi Colin,
the virtual Synchron Stage in MIR Pro will help immensely when you want to combine Vienna Instruments with Synchron Instruments. However, I'm not sure what you mean by writing "Synch solo strings" as there ain't any, up to now ...?
... or do you mean "Synchronized Solo Strings"? In this case, MIR Pro not only allows you to combine SE with Solo Strings, but you can also use any other MIR Pro Venue with great success, as you can simply turn off the "synchronized" aspects of the Solo Strings: This way you get the pure sound of the Vienna Instruments-based samples.
HTH,