For your specific issue:
VFTAX YTD as of 7/2 close: -0.08% is correct (Vanguard and Pornstar, err, Morningstar give that -- links below).
Yahoo Finance has -4.13% (this turns out to be as of end of May close, 5/29/20)
The reliable and wonderful Yahoo Historic Page tells me what I need to know to figure out this sh**:
https://finance.yahoo.com/quote/VFTAX/history?p=VFTAX
First to find total returns, look at "Adjusted Close", not "Close". Adjusted Close includes reinvested distributions (so comparing two adjusted closes gives the total return). Whereas Close is just the share prices.
Adjusted Closes:
12/31/19: 30.17
05/29/20: 28.92 YTD as of 5/29/20 close: 28.92/30.17 - 1 => -4.14%
06/30/20: 29.79 YTD as of 6/30/20 close: 29.79/30.17 - 1 => -1.26%
07/02/20: 30.14 YTD as of 7/02/20 close: 30.14/30.17 - 1 => -0.099% (call it -0.08% )
So wassup Yahoo Finance with the YTD of -4.14%? Per my recollection, that number is as of the end of last month (usually). But in this case, they haven't apparently updated their internals to what end of last month is from 5/29/20 to 6/30/20.
As for YTD thru 7/2/20 close, 30.17 -> 30.14, why I get -0.099% = -0.10%, while they (Vanguard and Pornstar) get -0.08%, is because adjusted closes -- a complicated calculation when there are distributions -- is reported by Yahoo to 2 places, but internally the adjusted close is calculated to whatever precision they do calculations at, and that precise number is used in their return calculations.
And no, on the Yahoo Finance page, I don't see any footnote anywhere that the "YTD" is as of end of last month (let alone what they think "last month" ) is. Grrrrrr.
It irritates me to death that they've had like 20+ years to work out the bugs ... oh, never mind.
Morningstar is my usual "go to" place for fund information. Plus Vanguard for Vanguard funds.
All's I use Yahoo for is the historic quotes -- it's the only way I know of that lets me figure the total return between any 2 dates of my choosing. For example November 8, 2016 to present. For another example, January 20, 2017 to present. (Election day and Inauguration Day respectively).
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Vanguard: even though the URL says "month-end-returns", the column header says "As of 7/2/20 YTD"
https://investor.vanguard.com/mutual-funds/list#/mutual-funds/asset-class/month-end-returns
Yahoo Finance: https://finance.yahoo.com/quote/VFTAX/performance?p=VFTAX
Yahoo Finance Historic Quotes: https://finance.yahoo.com/quote/VFTAX/history?p=VFTAX
Morningstar: https://www.morningstar.com/funds/xnas/vftax/quote