Most Gram-negative bacteria are resistant  because their outer membranes are impermeable to large glycopeptide molecules (exception: some non-gonococcal Neisseria species).

If combined with Pip-Taz can cover pseudomonas, Enterobacter spp, serratia spp, proteus spp., klebsiella spp, E. coli, H. Influenzae

An appreciable number of Neisseria isolates, including N. gonorrhoeae are inhibited by vancomycin in in vitro testing. However, the clinical utility of vancomycin in treatment of gram-negative infection is quite low.

Vancomycin is a glycopeptide clinically used against gram-positive organisms due to its mechanism of action in the bacterial cell wall. Overall, gram-negative bacteria are regarded as intrinsically resistant to vancomycin because of their impermeable outer membranes and different cell wall structure.

However, there are two gram-negative organisms which are theoretical exceptions with very limited, if any, clinical use:

1) Some non-gonococcal Neisseria species in which in vitro susceptibility has been shown.

2) Elizabethkingia meningoseptica (formerly Chryseobacterium meningosepticum and Flavobacterium meningosepticum) which is commonly associated with severe and difficult-to-treat cases of meningitis and sepsis. Adequate treatment for this organism is not really known but treatment with vancomycin alone or in combination with other agents such as rifampin or TMP/SMX has been successful in some cases in the past. There exist conflicting data coming from recent studies reporting a poor in vitro activity, though.

Many of the Moraxella, Brucella and Pasteurella isolates are sensitive to Vancomycin.

Elizabethkingia meningoseptica is a well know Gram negative that could cause meningitis in newborn and immunosupressed p atients, and is susceptible to vanco.

It's well-known that most Gram-negative bacteria are intrinsically resistant to vancomycin because their outer membranes are impermeable to large glycopeptide molecules (with the exception of some non-gonococcal Neisseria species).

Regards

Brevundimonas vesicularis is another vancomycin susceptible gram negative

Overall, gram-negative bacteria are regarded as intrinsically resistant to vancomycin because of their impermeable outer membranes.