TD Ameritrade offers free Vanguard ETF's. You could just deposit whatever cash you have left at the end of each month into a cash account at TDA and then invest in taxable ETF's at your leisure.
That's what I do.
so you can invest in Vanguard funds through TD Ameritrade? do you plan to move then to Vanguard later? sorry these might be dumb questions, still learning this world.
Absolutely. ETFs are funds that you can easily buy and sell like individual stocks. TD Ameritrade (and other online brokerages) offers about 100 ETFs which are free to buy and sell, and a few of these are the Vanguard funds we know and love:
BND - Vanguard Total Bond Market ETF (same fund as VBTLX)
VNQ - Vanguard REIT Index ETF (same fund as VGSLX)
VB - Vanguard Small Cap ETF
VIG - Vanguard Dividend Appreciation Index Fund ETF Shares
VO - Vanguard Mid Cap ETF
etc, etc.
If you are new and a fan of MMM, you are probably going to want to start by looking at VTI - Vanguard Total Stock Market ETF which is the same fund as VTSAX.
I referee soccer games on the weekends and pull in $50-100, which goes exclusively to my taxable investment account... basically exactly what you're talking about doing. All the benefits of Vanguard because I do own the Vanguard fund, but none of the hassles of an actual mutual fund like minimum amounts. You could transfer it to Vanguard once you have enough, but it probably isn't worth the trouble.
You will want to keep a log of everything you buy, date, amount, price, etc. because although TD Ameritrade does keep track of it all, I can't yet vouch for how smooth it is to report come tax time.