This value is presented for information only. As vector addresses are configured, this address is computed to be an appropriately aligned address such that all the dynamic vectors are contained within the maximum relocatable vector block size.
This VECBASE register can be either initialized by an LSP, or it can be written to dynamically. Either way, generally vectors will need to be built at the alternate locations using an appropriate LSP such that they can be appropriately loaded. See the Xtensa Linker Support Packages (LSPs) Reference Manual for more information.