In this month (December) adjustments, I had to use SPY to adjust my delta without impacting Theta and Vega. Buying 200 shares of SPY would cost about $28,000 at current market level. It requires a lot of buying power. So I played with the double long ETF for SPY: SSO and triple long ETF: UPRO in the TOS analyzer, in order to reduce the buying power impact during adjustments. I found SSO (~$60) appeared to be a nice replacement vehicle for the adjustments. It costs less than half of the SPY and can produce a SPX delta of 8.4 for 100 shares. So overall, the buying power requirement can be reduced by about a half using SSO. For some reason, the triple long UPRO could not be used in TOS analyzer as it does not show any delta value.
If negative delta adjustment is needed, I would expect the SPY double short ETF SDS would work similarly. It's around $55 at present. I plan to use these two ETF's as adjustment vehicles in the future.