Like the OP, I am relatively new to Vanguard. I spent much of the past 3 months ferociously researching investing, retirement accounts, etc., so it's still new to me, but I know way more than I did.
Here's what we did:
1. Figured out our risk tolerance. We're good with about 85% stocks and 15% bonds, even though we're relatively close to retirement age. We're both still working and can absorb some roller-coaster swings.
2. Created a plan based on our current ages, expected retirement ages, current portfolio amount, risk tolerance, current mortgage amount, mortgage interest rate, and home equity. Our plan is 85/15 stocks/bonds while we're working, and then reassess when we're not, possibly changing to 80/20.
3. Put most of the 85% into VTSAX, which, because it is an index fund that has all the stocks, is extremely diversified.
4. Put the rest of the 85% into a few other funds I gleaned from scanning both MMM posts & comments and from Googling "Vanguard index funds": VIGAX, VSMAX, VIMAX, VMGRX. That last isn't an index fund, but its performance looked interesting, so we took a chance.
5. Put the 15% into VBTLX, which, as an index fund with all the bonds, is also extremely diversified.
Remember: Roth IRA, traditional IRA, SEP, SEP-IRA, 401k, non-retirement -- these are all labels for types of accounts, which has to do with how much you can put into them each year, how and when you can access the funds, and how the profits are taxed.
Bonds, stocks, ETFs, index funds, mutual funds, REITs -- these are types of investments, which has to do with what you're actually buying. These are the things that go inside the type of accounts you choose. I think of it like stocks = 1 company, most mutual funds = several companies, index funds = lots of companies, and VTSAX = all the (U.S.) companies.
It is seriously worth researching the tax advantages and restrictions on retirement accounts. Six months ago, I didn't understand that it made sense to shovel as much money into those accounts as possible. (Our case is unusual, in that spouse is old enough to tap retirement accounts now, and I will be in a few years.) Basically, you want to keep enough money outside retirement accounts to live on until you can easily tap those accounts, and put the rest in to take advantage of the tax breaks you get.
Good luck.