Good point about the commissions. If you are going to buy Vanguard products (and I think you should), you can run your Roth through Vanguard. This will certainly avoid any commission costs. Disclosure: I'm a part owner of Vanguard (as is everyone who owns Vanguard funds. I just love saying that. :-)
VTI is a great place to start and is the corner stone of many excellent portfolios. The next step is education. I recommend starting here:
http://www.bogleheads.org/wiki/Getting_StartedAs you start to learn about asset classes, risk, and constructing portfolios, you will probably want to add some bonds and perhaps some international funds. Google "JL Collins" for more on these topics. He has an absolutely great series of posts.
One final thought: given back of envelope calculations on your income, you should plan to get the saver's tax credit in 2014. It is capped per-person, so you should also open up a Roth for your wife. She can contribute under the "spousal" provision. If you split your contributions over both people, you'll get some tax advantage.
Good luck!