Is silicone an adhesive? Question

Now this is an area I have a fair bit of experience in

The adhesive properties vary depending on the material you're trying to bond. e.g. PVC glue is great for balsa wood, but doesn't work on plastics.

Silicone, by nature, forms a nice flexible gasket when it goes off.

My approach would be to have the tube slightly larger than the hole, so that it compresses slightly and the elastic properties expand out to create a seal. Then apply silicone to the two sides of the 'seal' to make it water tight.

Many model building [ships/planes etc] hobby shops sell model glue that kind of 'melts' the two plastic surfaces to be bonded so that they then kind of mould together, and then the glue goes off.
[edit:] This is a similar to process to the way metals are welded together. The glue helps to weld the plastic together and then locks it in place when the glue settles/goes off.

Products such as Araldite and other 2 part epoxy resins are great for plastics. Some contact adhesives may also work. I imagine that most hardware/Walmarts would probably stock a wide range of adhesives. I'd just read the labels looking for ones designed for plastic/vinyl.

HTH.