"Vi devu ensaluti" (or simply "devu ensaluti") would be very odd. It would be an order, "Have to log in!".
"Devi" doesn't only mean "need to/have to/must", but also "should" (when it's "devus").
Additional information: in Esperanto you can use the imperative directed to yourself and in interrogative sentences. "Should I stay or should I go?" can be literally translated as "Cxu mi devus resti aux cxu mi devus iri?", but probably people will just say "Cxu mi restu aux cxu mi iru?" (it sounds more natural and it's much shorter). I remember that Evildea once created a survey where he asked "Kiel mi prononcu mian nomon?" (How sould I pronounce my name?) - the choices were "In English (as I already do)" or "In Esperanto". It makes sense because the answer can also be an imperative verb (- How should I pronounce? - Pronounce it that way = - Kiel mi prononcu? - Prononcu tiel).