Well here are my thoughts FWIW (a bit more than you pay me but only a tiny bit.)
Eyeballing the chart VFIIX prices has been $10 +/- $.50 for ~90% over the last 25 years. (The first few years of the fund were a roller coaster).
The price has remained stable despite some dramatic shifts in the benchmark 10 yr T-Bond (in 1984 is 12.5% and last year it dropped to 2.5%) .
Of course over the last 6 months it has remained above the 10.50 level but only marginally. The relatively high price of VFIIX coupled with the juicy yields was the reason I switched roughly 1/2 my bonds to High yield. The trader testosterone also kicked in.
Of course, I left my mom's much larger position in VFIIX intact cause at 83 she doesn't need to take additional risk.
So under normal circumstances I'd say as long as you can stand to see a 10% paper drop in your position (10.5x to 9.50) it should fine. More over even if the price drop the yields will increase and the monthly distributions should remain relatively constant, I spend the interest and the distributions haven't changed a lot (down 10% year over year).
Now obviously if interest rates rise (and most folks think they will) than any bond fund price will drop. The stability if GNMA price is enhanced because in normal environment the increased economic activity means that as more people move, more mortgages are paid of early, and the new mortgage pay higher rate. In a falling interest rates environment people don't even need to move just refinance which hurts the distributions of the fund.
Now all of my analysis is based on normal circumstances, which unfortunately probably excludes today's economy.
So I see 3 probable outcomes for VFIIX
- Continued recession interest rates remain low, fund price remains high and at 4.2% the yield is way higher than any other safe investment.
- Normal recovery, modestly higher interest rate, but demand for houses increases so fund distributions increase, and price drop modestly but total return is still positive.
- A severe bear market in US Government (backed) bonds. Concerns about the safety and reliability of US debt increase causing significant drop in the fund price. The big area of concern, I have with GNMAs, is while GNMA in theory are backed by the full faith of the US Government, it is important to remember that the first line of defense as GNMA bond holder is the assets which back them. This means a bunch of mortgage on US homes. How many of these home owners are underwater is a question which I don't think anybody knows. At what point will the US government stop pouring money into GSE to cover their massive losses is another unknowable questions.
I don't want to sound alarmist, and 1 year ago I would have laughed at a posted picture of tinfoils hats to anyone who suggested than GNMA were anything but the safest investment on the planet, but times have changed. I think #3 is very unlikely but no longer impossible.