Right - I stopped reading the Daily Mail a while ago and so I'm missing out on this drama!
Have I got this right, Pippa is marrying someone called James, who appears to have had posh parents but has earned his own wealth rather than inheritied it.
His brother is a reality TV twat called Spencer, and said reality TV twat brother is the best man.
Vogue is one of those TV personalities called a model to explain why they are always on various TV shows for famous people, she started dating Spencer after meeting him a couple of months ago - probably after the guest list was done, so she's not been invited. She's not a friend of either the bride or groom, so would only expect an invite on "family" category, but having dated for less than 4 months doesn't get you in the 'family' category for most people.
Harry on the other hand is invited, being a friend of the bride and groom, having been seen at various social events with both for years. He's been invited with his girlfriend who he's been dating for best part of a year, so a 'proper partner' not 'the person shagging this week'.
That one of the other guests is invited with his long term partner is seen as a snub to the brother of the groom's new girlfriend because you should treat someone as close family after the 2nd date or something...
If he wasn't the best man, then I'd agree it was not nice to give someone a "plus 1" invite in a 300 guest wedding, but the best man will have various duties, be sat on top table etc, so it's not like he needs a date to keep him company.