I went here last night, and the waiter said most dishes cannot be made without fish sauce, because it is part of their base sauce. When pressed which dishes do not have fish sauce, the waiter said that all of their dishes had fish sauce, even their curries. I ended up getting two appetizers for dinner (vegetable tempora with a peanut sauce and tofu with a sweet and sour sauce). The waiter had to check, but did come back saying neither sauce contained fish. My gf went through about 5 dishes before finding a dish they were willing to make without fish sauce (the red curry).
In the end, my food was fantastic while the gf's food was pretty bland. The staff was quite nice, but I would definitely not call them vegetarian friendly. There are many better places in Northampton for vegetarians and vegans.
Cons: not vegetarian friendly