How can you know? Look at the susceptability data. Higher MICs, even in the susceptability range, may warn of treatment failure. Here are some papers mentioned at SHM 2009:
In MRSA bacteremia vancomycin MICs of 1.5 or greater were associated with a 36.4% treatment failure rate.
This paper showed an atypically high failure rate in bacteremic patients regardless of the MIC, and a virtual guarantee of failure for MICs above 1.
For MRSA isolates with vancomycin MICs less than 0.5 µg/ml, vancomycin was 55.6% successful in the treatment of bacteremia whereas vancomycin was only 9.5% effective in cases in which vancomycin MICs for MRSA were 1 to 2 µg/ml.
This paper showed higher mortality in MRSA bacteremia treated with vancomycin when the MIC was above 1.