Do a booking and after selecting your flight options you go to the "Guest details & travel extras | Virgin Australia" page.
View the source and about 33 lines from the top you'll see "farefamily". It may be easier to search for this string.
The fare class is then indicated after the '='. e.g. - an "M" class flexi booking MEL-CNS:
Code:
...
var productElement = 'MEL-CNS_M_M';
var fareFamily = 'M';
var catElement ='MEL-CNS';
var priceElement ='329';
var qtyElement ='1'
var storeElement ='1';
...
... or a return in 'saver':
Code:
...
var productElement = 'MEL-CNS_U_U,CNS-MEL_Via-BNE_A_A';
var fareFamily = 'U,A';
var catElement ='MEL-CNS,CNS-MEL_Via-BNE';
var priceElement ='264,285';
var qtyElement ='1,1'
var storeElement ='1';
...