IMHO.. you have to get rid of the grass ...

i dont not think rototilling such will get rid of grass..

and if you are in shade.. under mature trees.. do NOT rototill ...

you really havent given us much info .. other than you want to do it ..

but one thing for sure.. i would not be doing it in july/august ... fall planting would be better ... so you have time to figure it all out.. and kill the grass i the meantime ... i would use roundup.. but there are many alternatives ... that include hard work ...

and a pic will really help us understand the situation.. e.g. .. how will apply deep water to recent transplants.. on a hill??? been there.. tried that.. usually failed wildly.. it can be done.. but its not easy.. aftercare is the key ...

and clay is NOT going to make it easy ...

this is all possible.. but success will take a lot of work.. and planning ...

BTW.. i dont care where you post.. but isnt vinca a perennial???

good luck

ken