How do you add an onboard ISP for an ATTiny85?
If you want a part that's programmable over USB, use a microcontroller with a USB bootloader, like the ATmega16u4/32u4.

Libraries like V-USB are intended for designs which are primarily a USB device. They have nontrivial resource and hardware requirements (especially relative to the ATtiny85), making them a poor fit for a design where USB is an infrequently used secondary feature.