How do I invoke a maven task without leaving vi?
Maven is no different than any other shell command:
:!mvn
You can :set makeprg=mvn if you already have a favourite key mapping for :make.
Can I get code completion?
Yes, eclim is great, a bridge between vim's editing efficiency and Eclipse's Java language-specific awareness.
How's the syntax highlighting?
More than good enough for a regex-based highligher.
You may want to consider tools for other vim+java purposes, like code templates (snippetEmu—default snippets suck, but customizability shines), searching for usages and going to declarations (eclim, grep, ctags), generating getters and setters (java_getset, or eclim), automatic imports (eclim). You might also need a java shell for quick experiments (the BeanShell, a.k.a. bsh).
