The greenhouse gas emissions of food products is a big deal – agriculture accounts for about a third of world greenhouse gas emissions.
As a rule of thumb, the production of meat from plants is inherently inefficient. Each time you go up a level in the food chain you lose energy. For example, in the case of beef, which is substantially grass-fed, about 50g of soya goes into creating a 100g beef burger.
Red meat is also plagued by other climate issues, as it is the source of so much methane and nitrous oxide, both far stronger greenhouse gases than carbon dioxide.
(Please note, it is only animals that eat grass or other cellulose-based food that produce significant amounts of methane in their flatulence and burps. Human flatulence is nearly all nitrogen).
However, exactly how bad you think meat is for the climate depends on many things, not only how it is produced, but how you divide up responsibility for things like deforestation.
It is hard to find figures specifically on veggie burgers, but the relative figures for meat, Quorn and soya-based meat substitutes in general are given in the graphic below.