Hello Astro,
I would think that yes SMBS (Sodium metabisulfate?) or citric acid could work to improve the shelf-life of vacuum packed vegetables. However, they may leech moisture from your product, resulting in a mushy texture. They also carry a flavor, which be unappealing to consumers if used in larger quantities.
How much your shelf life is improved, what the specific microbial risks are, and the subsequent control points that will characterize your process will have to be the result of a shelf life study and validation generated by your team. If you don't have a lot of experience with shelf life studies or food safety studies, 3rd party labs or consultant firms do exists to help you through the process.
As far as I know, plenty of companies vacuum pack raw vegetables without the use of any preservatives (much of fast food buys their vegetables in this way). I believe they rely more on a robust washing procedure and facility sanitation to package the product as cleanly as possible. The product still has a pretty short shelf life, but consumers appreciate buying a bag of shredded cabbage that is just that: shredded cabbage and nothing else added.