For rebalancing, the best option is going to be using a Vanguard mutual fund like the Total Bond fund (VBTLX). Let's say that stock prices rise and your 80/20 asset allocation becomes 90/10 so you need to sell stocks and buy bonds. It is an easy change within your brokerage to do this (sell a dollar amount of one fund and buying that dollar amount of a different fund) rather than selling stocks, transferring to Treasury Direct, then buying individual issues. Treasury Direct is great for buying I Bonds, but a mutual fund would probably fit your needs better.