I agree with what @D Stanley has already said. But would like to add a few points.
1) If you're just looking to add some bonds to your portfolio to balance out the risk Vangaurd does a total bond market fund that has a low MER VBMFX (MER 0.15%).
2) Deciding how much risk/reward you're willing to take on is a personal choice but will be informed by your risk tolerance, your age and the likelihood that you will need to remove your money early. Equities will always beat bonds over the long term so if you have the time to wait, are in a stable job, and no where near retirement then there's an argument to be made for 100% equities. Here's an article on how to decide your asset allocation.
3) If you like the idea of having some bonds in there but don't want the hassle of re-balancing your portfolio Vangaurd makes targeted retirement funds which gradually shift your balance of stocks to bonds based on the time you're planning on retiring. If you like the idea of the fund but want it to be a little more or less aggressive just adjust your foretasted date or retirement-- making it later will put a higher proportion of equities in your account, while making it sooner will increase the number of bonds. These funds do have the downside that they have a higher MER (because they involve more trading), but they are a good set it and forget it option.
4) If this seems overwhelming Betterment is an option. They're the only robo-advisor that I've seen that offers similar levels of return as VTSAX (though obviosuly at a lower rate than if you do it yourself since you're paying for their service). Personally, I think it's always best to manage these things yourself since if you're not confident in the investments you're making you probably shouldn't be investing. But they are an option that offers some neat services and let you set your level of risk-tolerance, etc...