The stem may not always be typically like what they are expected to be. They are modified to perform different functions.

Stem tendrils develop from axillary buds. They are slender and spirally coiled and help plants to climb.

Leaves are often modified to perform functions other than photosynthesis. They are converted into tendrils for climbing.

The tendrils of grapevines are homologous to the tendril of pumpkins as both are originated from the same part of the plant i.e. stem but have different functions. In grapevines, the function of tendrils is to climb while in pumpkin is creeping.

The tendrils of grapevines are analogous to the tendrils of pea because they have different origins i.e. stem and leaf respectively but they have same function i.e. climbing.