Normally I look in the menu outside, if there is none and you sit down to realised it's all vegetarian, I would walk out because they fail to inform their customers that the restaurant only serves vegetarian products.
After sitting down n giving me the menu and i saw it wasnt wat i thought, esp. since it was changed to new management without stating so on the net, i would have politely tld them i didnt realise the new menu and it isnt to my taste so i'll be leaving, if they had brought water or drinks over, i would have offered to pay for those. As long as you havnt ordered or been rude, i think its ok to leave at this time, after all, if you dnt like it nw, u'd prob. never go back to it, so it doesnt really matter if you walk out cos you'll never go again!!
i ll walk out when the waitors or waitresses dont give...i walked out once at some viet restaurant...we got a table..the waitor dropped off some menus and left...we waited for 30 mins before anyone came to serve us..but they only came to give us tea and told us they ll be back to take our order..so we sat there and waited..after 15 mins, we asked a waitress nearby to take our order and she said they she ll get the waitor responsible..we waited for another 15 mins..no one came..so we put down some tips for the tea and left..LOL...we wasted like an hour sitting there..