The simple answer is: don't. Don't store data as concatenated strings, use a separate table to cross reference your vendors with the products they vend. It seems like more work to set up (and it is) but it saves a huge amount of time later - as you are starting to see. It's a lot easier to change that now - before you have a mass of code and data built up - than it is to decide you need to to it later when you have a huge task to sort it out.
But...you can do it, if you must. It's just horribly, horribly messy:
Converting comma separated data in a column to rows for selection[
^]
Seriously, change your DB layout now and the queries all become simple JOINs (which SQL server is very, very good at processing - string manipulation is not its forte, and is very slow as a result)
Submit an article or tip