Agreed. Make sure you understand what you're doing before you start investing. The advice of stranger's on the internet is worth what you paid for it.
Here's info on stocks (
https://www.bogleheads.org/wiki/Stock_basics) and bonds (
https://www.bogleheads.org/wiki/Bond_basics).
The brackets I'm talking about are both federal income tax brackets and capital gains tax brackets. See the table at
https://www.nerdwallet.com/blog/taxes/capital-gains-tax-rates/. Remember to select the tab for your filing status (joint I think). You should also consider your state tax brackets (
https://www.tax-brackets.org/newyorktaxtable). Like most states (at least those that tax income at all), New York taxes capitals gains, dividends and interest the same as any other income. If you live in NYC I think there's another city tax too. BTW, I wasn't implying that you should move or that we moved for tax reasons. We moved for my wife's job and our tax picture probably won't be that different now.
If you invested in 1/3 in each small, mid and large cap you would be highly tilted towards small and mid cap compared to the overall market (meaning much more small and mid cap companies than if you invested in the oft recommended VTSMX/VTSAX). Without taking too deep a dive into it, based on the link I posted earlier (
https://www.bogleheads.org/wiki/Approximating_total_stock_market) I would be looking at doing more like 80% large, 5% mid, 15% small. If you look at the link you'll see that there's a suggestion (5th one down in the table) very much like that using Vanguard funds. Honestly, if you want to keep it simple I wouldn't be overly concerned if you just wanted to put your whole 401(k) in the large cap fund.
If you decide to invest in bonds (read this:
https://www.bogleheads.org/wiki/Asset_allocation) then yes, VBFMX/VBTLX should go in your IRA. Bonds are terribly tax inefficient (because the income they produce is taxed at the same higher rate as regular earned income), so that's always the right choice.
I know JL Collins (Simple Path to Wealth author) isn't big on recommending international investing, but you should make an informed decision on that yourself. I would encourage you to consider making it 20-40% of your stock holdings. That would mean investing in VGTSX/VTIAX. Here's a good place to start:
https://www.bogleheads.org/wiki/Domestic/International. If you decide to invest in international, it might be better to put that in your IRA, but you might not have room if you invest in bonds. Putting international in taxable would also be totally fine, and if you're in a low tax bracket could even be preferable.
For the simple answer (still requiring research for the "as much as you want" part), first fill your IRA with as much VBFMX/VBTLX as you want, fill the rest with as much VGTSX/VTIAX as you want, if there's any room left in your IRA invest it in VTSMX/VTSAX. Next put and remaining VGTSX/VTIAX you want in taxable, and invest any remaining taxable in VTSMX/VTSAX. Invest your 401(k) either 80/5/15 in large/mid/small cap, or 100% in large cap and consider this entire balance the same as VTSMX/VTSAX when calculating your other percentages.
In case you haven't noticed, almost all the links I'm posting are to the Bogleheads wiki. It's an excellent source for just about any finance related topic you can think of.