I think there are two thinks you need to think about while coming up with your price.
How much work went into the vector originally? Is it a very complicated piece, or a small piece? Obviously you could give a discount for something you still had 'laying around' but you still put time and effort into it.
What is the purpose of the 'screens' you say they are using? Is this a commercial building? (ie, a restaurant, hotel etc) or will they purely use it as decoration for their office? If they will be (indirectly) making money with your artwork, you should keep this in mind while coming up with a price.
Depending on these factors, you can charge your hourly rate and a percentage for commercial use, possibly affected by the scale of the commercial use.