They are not "general" Ts&Cs, they are specific to you, they say that changes *may* be possible, which (and how is this not obvious?) mean they might NOT be.
What the exact conditions are for a change to be allowed will rely upon a huge range of conditions, many of which will be between Vayama and the airline they bought the ticket from at the time, and others will rely on when you bought teh ticket, when you want to change and to what, what agreements Vayama has in place with that airline at the time you ask for the change, and so on. Hence they say, a change *may* be possible.
If you wanted a flight where changes WERE (not "may be") allowed, you should almost certainly have booked with someone else, which in 99% of cases means, the airline. But there, you can choose both non changeable, and changeable (at extra cost) ticket types, I've done this in the past.
You however chose the very cheapest ticket type (I assume), didn't bother to check that changes were allowed for sure, ignored the part about changes only "may" be possible and presumably took that to mean they WERE possible and then complain.
Third parties like Vayama and similar sell cheap (sometimes) tickets which are more restrictive. It costs more to change and a change may not be possible.
All this was in black and white right in front of you when you booked. You just chose to ignore it and behave as if "may be" means "is"