Hello & welcome to VFlyer.
To begin, Virgin Atlantic don't have a First Class cabin. They have an Upper Class cabin which is comparable to business class on most carriers. That said, with the fantastic Heathrow check-in area, Clubhouse and generally great onboard service, it will certainly feel like a First Class product. It's certainly my favourite carrier to fly West. </Pedanticism Over>
VS and DL flights are released 335/334 days before departure. NYC is their most frequent route so I'd doubt you'd have trouble achieving two UC rewards seats. If there's an ounce of flexibility with your travel dates, I wouldn't be staying awake trying to book tickets. I need to caveat that with Easter falls on April 12th. When is your ideal travel date(s)?
With your travel date in mind, if it is around the Easter break, you will fall into peak ticket pricing. This is 57,500 miles rather than the usual 47,500 miles per passenger. It will also cost around £450 in tax each.
A final caveat, Virgin Atlantic don't promise to release any flights as reward flights as some other airlines do. Don't get hung up on this - if there is one place you can often get a reward flight to, it's NYC.
You get great flexibility when booking reward tickets. For £30, you can cancel or change your booking. I mention this as you may originally book for an earlier or later date than you'd ideally like, but when your date or time or aircraft choice appears, it's a relatively small change fee.
Unsure how familiar you are with Virgin Atlantic, but there's a few aircraft on the NYC route. Currently mixed between A330, B787 & A340. This will probably extend to the A350 by the time you fly (note: not by the time you book). Whilst it sounds somewhat insane to book your travel on a certain plane type (it can and often does change), a lot of us do. There's a recent thread discussing this... Whilst I quite happily fly the A340 as I prefer the bar layout onboard, a lot of members opt for the A330 or B787 for the better in-flight entertainment and generally better-looking cabins. You've got quite a while until you're booking, so you have plenty of time to read, watch YouTube and make your own mind up. Whichever aircraft type you're on, the food and service will remain the same.
Welcome to VFlyer!
Mike