As far as I know V-USB and software serial are two completely different things. I believe V-USB is code that allows a microprocessor to connect directly to a USB system - the same role that the FTDI chip does in an FTDI cable.
Software serial, on the other hand is code that allows a microprocessor to behave like a USART and send and receive data in the same way that the old PC serial ports used to do.
Normally you would use Software serial to allow a microprocessor to connect to a PC via an FTDI cable or equivalent - in other words you need BOTH.
It is by no means impossible to send or receive serial data with an Attiny provided you keep to a sensible baud rate. I wrote some code that works at 2400 baud on an Attiny45 running at 1MHz.
...R