Congrats on the windfall (and for wanting to invest it rather than drive to Vegas in your new leased Caddy Esplanade to paaaarty!)
The question "What should I do with a windfall $?" is a great test we can all do to check how happy we really are with our asset allocation. If the money is a relatively small amount* (compared to your FIRE target) you should have no problem simply adding it to your 'stach in line with your planned (and preferably written down) asset allocation and overall investment & tax optimisation strategy.
As per the stickies, you should know your investment order as well as the asset types you want. Typically that means ensuring you first get the free 401k match (if there you have access to one), then use all your IRA deduction, maybe HSA, then into taxed accounts.
In your case, your 'stach seems to be still at the stubble stage (my assumption), so I can imagine it can be a bit more of a quandry balancing planned diversification vs getting some funds from 'Investor' to 'Admiral' status to lower fees for example. At the beginning stage it's not really a huge issue that your detailed allocation is perfect - all VTSAX or some % of international and/or bonds if you're that way inclined. IE If your desired AA is say 70% US equity, 20% International and 10% Bonds, but you don't have at least a 30k 'stash, you will have difficulty doing that in Vanguard until you can meet the fund minimums. So what. At this stage just getting it saved is what counts more IMHO. Either stick it all in VTSAX until you hit a level that supports more diversification, or consider putting it all in a long time horizon life fund (say 2060 or something so the bond % is low).
WRT your specific question I would not go for the high yield but keep to VTSAX which gives you low cost with massive diversification wrt USA. I like to add a % skew towards Small Cap in my equities (more growth in the good times, more volatility tho') and have bonds and REITs. I stick with the advice of Bogle and JLCollins that there is no real need to add international equity as USA already has large international exposure indirectly.
If you are young and some time away from FIRE, it's hard to beat just regularly plugging it into 100% VTSAX... meanwhile ignore the market and never sell.
If 100% VTSAX (or equiv) is your asset allocation, but the thought of putting a big lump sum into the market makes you uncomfortable - that to me means there's an underlying issue with your decision on AA and risk tolerance.
*if it's a HUUUUUGE amount, then blindly sticking it into your current AA may not be the right move. If the extra money takes you from the situation of being in pure 'stach accumulation mode for years and straight into full FI and then some, that would probably be a good reason to adjust your AA accordingly, by adding more bonds and looking at RE for example. You can think about that on your way to Vegas of course, while your boss reads your resignation letter. #niceproblemstohave
