If you really want, I don't see why you couldn't go dagger with DL T5, Vistani C4 and some 15+ points into Assassin for the rest. I don't think the crit profile is worse than Assassin/Vistani, since you get +2 mult from DL T5 and +1 range from Vistani C4. You could even get most levels out of Rogue for the sneak attack.
By end-game , I don't think the build is that much weaker DPS-wise than pure assassin, depending on how many rogue levels you lose out on. You are mainly giving up a few sneak dice as well as assassinate + up to 30MP from Assasinate T5. But you get wings, the roar (!), extra feats and some imbue. I don't think the DL tactics is as useful on a dagger rogue, but it does not seem like a bad compromise. EDIT: Maybe you lose out on movement speed as well.
The main issue I see is that it likely has to be STR-based to make use of the roar. Or CHA via Illusionist, but then you won't have much points over for Assassin. Not that you gain much from Assassin APs anyway, it's mainly the rogue levels you want. Regardless, by not having DEX as main stat means you have to put some effort into ensuring that your Evasion reflex saves are high enough. Making it all work may require some fiddling. You could also go with DEX but then your roar is useless and I don't really see much benefit of going DL.
You could also just go pure DL/Vistani with daggers, that is probably easy to build and you get all the DL cores at the cost of Rogue sneak dice. DPS is lower but easier to build and play.