Portable, public domain, {v,}sprintf?

[Jeffrey C Honig]

I have an application I'm trying to make as portable as possible.  I've
become quite attached to using vsprintf (sprintf with varargs) but that
is System V derived and is not available on most BSD systems.  Has
anyone written a portable vsprintf that is available without
restrictions, or with just the Berkeley Copyright? Something close that
I can convert to use varargs would be ok too.  Does 4.3tahoe still use
_doprnt written in assembler?

My code is distributed with only the Berkeley Copyright and portions of
it are used in commercial products. 

Thanks.

Jeff