Atmega Standalone, Running without crystal oscillator
When using the simple RF transmitter receiver modules that don't include any protocols of their own like Virtual Wire is designed to work with jitter on the received data can be a problem. RF Monolithics have a good application not called Unique considerations for data radio UARTs that goes into some detail on this and it shows how to use a PLL (phase locked loop) to overcome the problem. The received data is over-sampled and when the received data edge is detected it feeds back into the loop so that the jitter is compensated for.

Virtual Wire uses a similar technique and samples the data eight times per second so an initial clock error of 12.5% would only be causing the data to be shifted by one bit which the loop would quickly compensate for while it was receiving the preamble. Then the RC oscillator will mainly drift with voltage and temperature changes which will be negligible over the transmission period so I don't think you'll have any problems at all.

For a normal UART that doesn't have that PLL scheme I normally like to keep errors on both ends at under 2% otherwise you can run into problems which is what the above is talking about. That's why a normal UART doesn't work so well with those sorts of RF modules that can have quite a bit of jitter and that's one of the things Virtual Wire is designed to get around.