I started with Devrim Gunduz' psql.vim at http://www.gunduz.org/postgresql/pgsql.vim, but very little of that remains. This is PG highlighting the way I use it. It may be useful to you, it may not. I suspect it will work very well for most, and if not, it's not too hard to fork.
Some improvements:
CREATE FUNCTION foo(_x INTEGER) RETURNS INTEGER
STRICT IMMUTABLE
AS $$
BEGIN
RETURN _x;
END;
$$;
The text inside the $$ delimiters will be highlighted. Of course, the syntax highlighting is always more PostgreSQL highlighting, since that's all I ever use. (I've considered conventions such as
I've done a good, but probably imperfect job of adding far more keywords and trying to get them to work contextually.
One of the things I've always liked about Microsoft's SQL Server is the @ sigil it uses for variables. This makes them stand out and prevents collisions with field names. Sadly, PG has no such sigil, and its parser won't allow you to use one. As a compromise, I always prefix my variables with an underscore, e.g., _foo. If you do this, you will get syntax highlighting for variables. They will stand out nicely, depending upon your color scheme.