The Perf trill patches are one of the best things about Vienna....they are NOT just for trilling.....If you have any legato passage that goes back and forth between two notes....even 8th notes.....so C, E, C, E, C, E you would use PerfTrill as it will trigger a different legato sound each time thus getting rid of the machine gun effect....
PS: Keep in mind that this only worx up to a Ma4th so playing C, E, C, E,C,E will work and playing C, F,C,F,C,F will not as it only worx up to Ma4....hope this is clear....
SvK