I used Vacationstogo once. I have no real complaints with them, but nothing terribly good other than that their site makes it easy to track cruise costs. They used a delivery service that put our documents on our porch (rather than inside the screen door) and they were destroyed by a neighborhood dog! I did insist that VTG cover the cost of replacement.
VTG prices are exactly the same as
DCL, and there is no incentive to using them (such as an onboard credit). The reason that VTG looks cheaper is that it doesn't include all the taxes and includes no "extras." If you look up a price on DCL, it will include all taxes, transfers, and insurance (you can then make appropriate clicks and remove the costs of transfers and insurance).
I'd recommend using a TA that offers an onboard credit or other incentive in addition to excellent pricing and service. There are several of these...
Dreams Unlimited and All Seas Travel are on that list.