Does booking months in advance make a difference?

Two trips coming up.
First will be to Las Vegas around the 9/10th March to the 20th. Either flying LGW-LAS or LHW-SFO-LAS.
Second will be LHW-HKG-SYD-LHW or whatever.
Can't book the second yet as I don't know the exact dates we're going and also few dates not available yet.
I could easily book the flight to Las Vegas this week for £750ish in PE (K), but would it make a difference if I wait a couple of months?
Am I likely to find, especially on this route, that there won't be any seats available or the price rise/falls depending on demand, operational costs etc?
Also balancing this with the fact the full fare comes off my credit card months in advance (unless it's 0% interest - although I could pay on another card, then transfer the balance to the Virgin CC and get 0% for 6 months, but probably lose out on the 750 FC miles)...
This is the major thing that's bothering me about the SYD flight. Would ideally like to be flying UC but £3k is a lot to come off in one go well in advance........shame with Virgin you can't pay say a 20% deposit and the remainder a month before travelling like high street travel agents do.
First will be to Las Vegas around the 9/10th March to the 20th. Either flying LGW-LAS or LHW-SFO-LAS.
Second will be LHW-HKG-SYD-LHW or whatever.
Can't book the second yet as I don't know the exact dates we're going and also few dates not available yet.
I could easily book the flight to Las Vegas this week for £750ish in PE (K), but would it make a difference if I wait a couple of months?
Am I likely to find, especially on this route, that there won't be any seats available or the price rise/falls depending on demand, operational costs etc?
Also balancing this with the fact the full fare comes off my credit card months in advance (unless it's 0% interest - although I could pay on another card, then transfer the balance to the Virgin CC and get 0% for 6 months, but probably lose out on the 750 FC miles)...
This is the major thing that's bothering me about the SYD flight. Would ideally like to be flying UC but £3k is a lot to come off in one go well in advance........shame with Virgin you can't pay say a 20% deposit and the remainder a month before travelling like high street travel agents do.