Vexlio comes with a minimal LaTeX bundle that is used to render the equations. The input you give in the edit window is automatically placed between $ signs to enter math mode. Currently the preamble is not customizable but you are not the only person to request this.
Right now there is no "official" way to use a local LaTeX installation, although that may become possible in the future. An untested workaround might be to symlink the "tex.install" directory in the Vexlio.app bundle to your local LaTeX installation. Please note I have not ever tested this and things may break.
PS: Any other questions in case I don't see them on this thread can be directed to the email address in my HN profile.
Now Linux please! This would be a killer app