vertical displacements have been heavily discussed in some topics (mostly here). My personal conclusion is that there is no standard way of doing it. There are many approaches, most of them require computing which cannot be done entirely in SNAP (e.g. vector or matrix based calculations).
These publications give nice summaries on the topic:
- Wright et al. (2004): Toward mapping surface deformation in three dimensions using InSAR
- Hu et al. (2014): Resolving three-dimensional surface displacements from InSAR measurements - a review
- Foumelis et al. (2016): Vector-based approach for combining ascending and descending persistent scatterers interferometric point measurements