It is possible to use Vaseline, petroleum jelly, or any other lubricant on the feeder to make it slippery and hard to climb. However, this is not a practical solution, nor is it durable.  First, using a lubricant may cause wild animals (including the birds) to have health issues or even die due to the lubricant that sticks to their fur or feathers. Moreover, the birds may ingest the Vaseline while feeding, and end up with serious problems because of higher exposure to chemical elements. Therefore, using Vaseline or any other lubricant is not recommended. Using lubricants on wild bird feeders is not practical either. They will be washed out in a short time because of exposure to the sun, dust, rain, etc., and the whole process will need to be repeated.  All in all, the use of Vaseline is not a solution to the squirrel problem. As a matter of fact, The Center For Wildlife in Maine has pointed out that the practice of using lubricants to get rid of squirrels is discouraged since it causes more harm to wildlife, including birds.