Here in UK adultwork allow us to advertise our services but vivastreet wont
Well, you'll never get a straight answer out of Viva about it but my assumption is (and this is based on my experience back in the day working for paper advertising)
Vivastreet is a classified ads directory which carries ads for jobs, cars, secondhand furniture, baby equipment, What's On, etc etc etc. Those advertisers periodically get all sniffy about advertising "right next to" an escort ad, and they threaten to pull their ads. So rather than Viva taking the adult decision to tell them to fuck off, they instead make all the escort ads as vague as possible so they can say "Hey, it's just companionship, if the girl decides to offer something else that's up to her!"
Adultwork is purely adult services - escorts, cams, pics, films. They have no "civvy" advertisers or clients to worry about. Nobody is ever going to complain that they were looking for a new pram and suddenly they got a pair of tits in the face.
I used to work for Friday Ad and they operate the same way now. I managed the team that did the escort ads and I used to have great fun dealing with the Colonel Mustard types who phoned up to harrumph at me about "Mature masseuse available for firm massage".
Him: You know it's not just a massage.
Me: How would I know that? Have you been to see her?
Him: Harrumph! Of course I haven't!
Me: Then why do you think she's offering something more than massage?
Him: You know why!
Me: No I don't, can you explain please?
I used to love imagining their faces getting redder and redder and steam coming out of their ears