You could ask the client and hope they know what a vector logo is or you can take matters into your own hands.
Brands of the World
The first place I check is Brands of the World. They have a huge collection of vector logos free to download with an account (also free).
If it’s not on Brands of the World I go to Google and type in the name of the company or business followed by .pdf usually find an annual report or a sell sheet. Open the PDF in Adobe Illustrator and you can pull out the logo in vector format.