That would take some consulting with the client and decide with they want. I would personally suggest to the client to remove the vines as they could potentiality case damage to the siding, and invite bugs and pest up the side of the house. I would pass along my concerns to the client and proceed accordingly.
I explain to the customer that they will need to be removed if and when they decide to paint the house and that the painter should remove them and be responsible for the prepping of the surface for paint. I also let them know verbally and in writing that we are not responsible for any damage they may have caused and that show's up after the cleaning.
And i agree with Jeff, its the little feet that are always the problems....
If you're able to, the multiple application of wash mix will cause the little "suckers" to degrade over time however, the customer should be informed before the work begins so they know what to expect once the job is finished.
I hate vines too and agree, they shouldn't be allowed to grow on the siding in the first place.