Pvftormd will fail and also tell you what the desired sampling rate is for your modem. Use pvfspeed -L to see the list of supported modem types. The modem type is case sensitive. The next parameter that can be seen here is 4, which is the number of bits per sample. Possible values for your modem will be shown in the list of supported modem types.
More bits means larger rmd files but better sound quality.
The above series of commands can be compressed into a single command line as given below. Vgetty offers you the option of using multiple greeting files. Create any number of files and place them in the messages directory. Only one filename per line. Vgetty will now use any of the files specified in there as the greeting message.