I have some 1080 48 fps files that need to be mixed with some 60 and 30 fps files ... and rendered to 30 fps as final product..... any recommendation for the 48 fps material ? or just mix with the others on my timeline (60 and 30 mix very well, but 48 is new for me.....)

regards

Theoretically, as long as Vegas knows it is a 48-fps file, it should handle it, and mix it with the other files, just fine.