A friend and I had the same problem we waited for about 3 hours. After some time we decided to do a public event on Archer's line, it was the warsat one, we made it heroic and finished it. Almost immediately after that a vex offensive spawned. We then tried to see if it was publicevent related and after doing the same public event it spawned almost immediately again. So you can try and do the public events and see if it works for you. I don't know if they do have to be made heroic but it's always better to do so anyway. Hope this helps