I absolutely echo previous the responses regarding etiology of the hypotension at hand. As a general statement, sedation is not a "one size fits all" type of therapy (not much is in critical care, minus celestial transfer).
Lately I'm finding that with patients that don't evolve well with either precedex or diprivan have done better with combination of both. If the goal is vent. weaning I have a tendency to go heavier with the precedex (our hospital / anesthesia protocol allows for up to 1mcg/kg/hr and suppliement with diprivan, vs. more of a generalized sedation where the roles may be better reversed (heavier on diprivan with precedex as a suppliment). Also bear in mind that precedex has a tendency to potientiate co-administered central nervous system depressants. I've read various studies / drug-rep's that have stated by a factor of 1.5 - 2X. Not sure if there is a solid number to state, but It's something I am aware of while medicating my patient.
Now add narcotic / opioid therapy. Fentanyl may be ideal due to it's lower incidence of induced hypotension / decreased histamine release vs. morphine / dilaudid, and shorter half-life, but it's far more expensive vs. the other 2 mentioned. Add any hepatic / renal impairement to murky up the waters as well.
Ativan / valium (old school) are also a valid option but they have a far greater half-life / clearence, especially with long term usage. I personally prefer to use them as PRN suppliments, but will absolutly advocate for a ativan drip if events / needs dictate.
Now lets talk about your stated dosing of dopamine / Neo. If your not getting the effect you want (or need), it's time to advocate for different flavor (similar to sedation no?). Your maxed on dopamine, and very maxed out on the traditional dosing of Neo. It's time to add Levophed and possibly look at the usage of vasopressin (last on, first off). However, a lot needs to be considered with this, ie: etiology of the hypotension (septic, post bypass pump, etc, etc, etc). Whats your heart-rate at, especially considereing your at 20mcg/kg/min of dopamine, volume (you stated that volume was not an issue, but I'm mentioning it as a general consideration).
I'd say it's time to try a few different flavors to get your BP in line (inferred), then worry about sedation. (I'd would have advocated of precedex and suppliment with diprivan / fentanyl)
And this is just over the few minutes I have left on my break to answer.