Thanks to Gian Luca who found the problem, I've published a bugfix for VirtualWire for PIC to correct an initialization overflow when calculating the constant for timer0 interrupt rate. The problem occurs either when the oscillator frequency is high enough, or the bitrate is low enough, or both, to overflow the TMR0 value without using the prescaler.

The fix was tested with a 10MHz oscillator crystal and a very low bitrate of 50 bits/s.

Read more at the project page.