Let's just have a look at the spread difference between GBPUSD and EURUSD from last 7th July,
GBP/USD = 1.5674 - 1.5449 = 225 pips
EUR/USD = 1.1195 - 1.0807 = 388 pips
Receive a large spread via volatility swaps, EUR/USD volatility has limited room to increase from current levels as the market already discounts a disorderly fall and Greece issues have also been settled temporarily.
On the flip side, The EUR/USD vs GBP/USD volatility spread is very wide on a historical comparison and in addition to that Fed's big event is nearing.
Since currency swaps are motivated by competitive advantage when we you look at the volatility difference of the ATM contracts in the above nutshell, hence we recommend trading the spread on the 6M tenor.
Currency swap contracts would require principal specifications in each of the two currency crosses. Such principal amounts are usually exchanged at the beginning and at the maturity of swap contracts.
Long 6M GBP/USD volatility swap, while shorting 6M EUR/USD volatility swap, indicative bid: 2.2 vols for the spread.