Are the CDs in IRA? If so you may want to consider buying the TIPs directly. As low as Vanguards expenses (.11%) are the absurdly low rates for TIPs means you are making very little money.
When I bought TIPs and IBonds in 2000/2001 with real yields between 3.5 and 4% they were good deals At 2-2.5% they are probably ok, but it does make it difficult to achieve a 4% SWR. At the current 0-.75% rate I don't see how can one afford to have SWR rates much above 2.5% if say 50% of your money is TIPs with 30 year at .75%.
Let's put it this way there is no way the returns of VAIPX going forward are going to match the last 3 years.