Using USI for implementing USB

Would it be feasible to use the USI found on attiny cpus to implemen a usb interface of the same type as V-USB or similar?

Well, the V-USB approach definitely works on the ATtiny85 as per the DigiSpark and Trinket. Are you asking if you can do USB-Serial in a normal sketch, outside of the bootloader?

Both USB serial and maybe even HID. I have started to to study the VUSB code but it is a lot to take in. My thought was that som eof the time critical low level routines could be made more effective by using USI

This guy got USB-serial working on an ATmega, you may be able to leverage his code for ATtiny:

http://rayshobby.net/?p=7363

The bottom of that page

http://rayshobby.net/?p=7363

Has code for the Tiny45 as far as I can tell ie there is a zip file called hid_serial(Modified for tiny 45)

Sorry guys, i couldn't open that page

http://webcache.googleusercontent.com/search?q=cache%3Ahttp%3A%2F%2Frayshobby.net%2F%3Fp%3D7363&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=sb

https://github.com/rayshobby/hid-serial

http://rayshobby.net/software/hid-serial_tiny45.zip