Vancomycin inhibits bacterial cell wall assembly and linezolide works by inhibiting the initiation of bacterial protein synthesis. Gram negative bacteria have bacterial wall and ribossomes,
Vancomycin is too big to diffuse through gram negative outer membrane via porins so access its target in periplasm is more difficult. I don't know about linezolide but I guess it is also linked to diffusion rate to access the target...
Linezolid is a good substrate of mdr efflux-pumps such as AcrAB-TolC in E.coli resulting i a high export rate of linezolid and consequently a low intracellular concentration. E.coli deletion mutants lacking the negative control elements of acrAB, such as the MarR repressor show significantly increased linezolid susceptibilities.