Yes, vegetable oil works. The green goo is a waxy substance impervious to soap. The oil bonds to the wax, and then the soap can bond to the oil and wash it away along with the wax. I just wash it off with Dawn dishwashing soap, followed by thorough rinsing.
I have a fruit bag that is set aside exclusively for Elderberry. I cleaned it the best I can, but it isn't perfectly clean. One of my 5 gallon fermenting buckets was so bad that I just retired it to the garden. They don't cost very much. But my carboys and other glass equipment cleaned up very well. I planted three Elderberry bushes, so hopefully in another year or two I 'll have a good harvest.