Long story, but short warning: Do not book a flight, hotel, etc on Vayama unless you are 100% sure you will NOT have to change the booking.
First, some background: Normally, when you want to make changes to the flights for an airline ticket you bought from a travel agent, before you fly the first segment(s), you must call the travel agent. After flying the first segment(s), you call the carrier that ticketed the ticket. (Example: If the ticket number starts with 016, this is a United ticket and - normally - United takes over the itinerary from the travel agent after you fly the first segment.) This is true even if all the subsequent flights are on a different carrier than the one that flew the first segment(s).
But, Vayama often buys airline tickets and hotel rooms in bulk. They buy these tickets on popular segments (think NYC-LAX, SFO-NRT, ATL-FRA, etc) at prices below what any individual traveler can get. Then, when those segments are added to an itinerary you want to buy (eg: NYC-LAX-SAN, SFO-NRT-OSA, ATL-FRA-MUC, etc), they upcharge them significantly - meaning: more than the 3% or 5% that travel agents usually get in commission.
The problem is that, if you need to make any change to such a ticket, you can never go to the airline. Even if you have flown all the segments except the very last one, even if you are a top-level platinum super-duper whatever flyer, your airline will send you back to Vayama. Thats Bad for you No 1.
Then, when you call either of the phone numbers on the Vayama website (meaning: the 877 toll-free number or the 650 San Jose area code), option 3 in the auto menu tells you that, to make changes to your ticket, go online. But, the online web service refuses to change your ticket, telling you that you have to call the 650 area code number. So, you get the run around - thats Bad for you No 2.
By the way, the 877 toll-free number has an interesting personality quirk - it drops calls that have gone on too long. Specifically: If your call exceeds approx 30 minutes (which, of course, Vayama is paying for - its a toll-free call to you, not to Vayama), the call is dropped - even if you are the next caller in line. Thats Bad for you No 3.
If you are clever, you realize that the best auto menu choice is DO NOT CHOOSE ANY OPTION. This brings you - eventually (the wait this evening was over 40 minutes - twice!) - to a live person. The live person will tell you a bunch of interesting things but, the bottom line is that you cannot cheaply make any changes to these tickets other than the date of travel. That is: If you want to change the route, or an airline, or even the time of day to travel, the fare difference can be staggering. This is primarily because you are exchanging a bulk ticket for an individual ticket, and you will get hammered. Thats Bad for you No 4.
If the above four items were not bad enough, heres the clincher. When you want to change your itinerary, most airlines charge a change fee of anywhere from $100 to $300 or more. Whats so ludicrous is that this fee is charged for the Customer Service person pressing a button on the flight sequence you have chosen. Of course, so they say, the airlines have to pay those Customer Service persons to provide that customer service. (Question: Then why is there a change fee still charged even when you make the changes online?) But, weve all gotten used to the brave new world where even using the bathroom on a flight can be chargeable offense. (Spirit Airlines, were looking at you!). However In the case of Vayama, they collect that change fee for the airlines and they also change their own change fee! That is, you get the thrill of paying two changes fees for one change! And, this is not the $39 change fee charged by American Express for a US-based AmEx representative - this is a full $100 for a person who is clearly not based in the USA! Thats Bad for you No 5.
So, Im done with Vayama. I have too much variability in my flights to want to endure the flight change process even one more time. Again: If you want to book international flights and you are 100% sure you will not change the itinerary (eg: for a family summer holiday), Vayama is among the best travel search engines providing the most offering airlines and route options. But, if you pay for flights and change fees yourself, and you even occasionally change your itinerary, DO NOT USE VAYAMA.