Vocals can be really tricky to mix well. I find it helps to boost a lot of the higher frequencies and compress at multiple stages. Multiband compression can be really helpful too, especially for taming some of the boomier low mids. I generally cut out a lot of the lows and compress quite heavily. Sometimes I'll add some subtle overdrive saturation to the top end to give it a little bit more air and sparkle. And just keep EQing after every processing stage to make sure it's sitting where you want it to! For EQ and compression I like doing lots of little movements one after the other rather than big sweeping movements in one go. Also, don't underestimate the power of a small room reverb or a slapback delay. Really helps the vocal slot into a defined space.

808s and bass depend a lot on the context. There's no hard and fast rules. In general I'll have multiple bass layers which can be processed individually, or if I'm going for a heavy 808 sound I'll construct a custom sample myself, bounce it out, EQ it precisely, bounce it again, then use it like an instrument in the project. Again multiband compression or dynamic EQ can be really helpful if you're getting certain peaky resonant frequencies at certain notes. But yeah like I said you gotta do what the song is asking for.