Hello Mark
Having never flown Virgin, I'm not familiar with their baggage allowance.
However, below is a link for AA baggage allowance:
http://www.aa.com/content/travelInformation/baggage/baggageAllowance.jhtml
This is the link for US Airways (American West) baggage allowance.
http://www.usairways.com/awa/content/traveltools/baggage/baggagepolicies.aspx
Below is a link for general baggage allowance for most airlines from the UK - USA.
http://www.agents.usairtours.co.uk/important_information/general_baggage_allowance
One issue Mark, you state "Unfortunately the itinerary does not state the full flight code, the code for this flight and the flight from Cancun to Mexico (which incidentally has an 'AA' against it - American Airlines perhaps?) Are not complete."
First thing "AA" is American Airlines.
Flights seats can be sold 330 days in advance, as your trip is well within this time scale, you must have seats booked on a system somewhere.
What tends to happen, although I'm not saying this is the situation in your case, Airlines over book seats, so, when agents try to book seats, they are held within the system, and your confirmed flight is only confirmed very close to the flight date.
For example, go onto any Flight website and enter a dummy flight itinerary for sometime in November this year, the itinerary will give you the official flight details, therefore, I'm unsure why you cannot obtain your flight details.
Reply in a pm to this message if you wish, One thing comes to mind, have you booked your complete trip through Virgin Holidays?
I don't believe Virgin have a code share, unlike most US carriers, which is why you have not received confirmed flight details within the US.
What you have confirmed is LAS - PHX - CAN this is a regular schedule, operated by several USA airlines.
Call your agent back again, and ask what flight you are booked on, if they cannot tell you, ask why not.
I travel on a fairly regular basis to the USA, and book well in advance, with confirmed flight details.
The fortunate thing Mark, sorting it out this far in advance, saves all the aggravation later on.
HTH
Driver02