How do plant cell vacuoles form?
Does the plant cell vacuole form by the invagination of the cell membrane?


Yes, in part, but the complete answer is way more complicated than that.

Endosomes (the vesicles formed by "invagination" of the cell membrane, as you said) are responsible for (part) of the biomembrane of the cell vacuole. However, some biomembrane comes from the Golgi-ER system, and almost all proteins come from ER (having passed by the Golgi apparatus).

Have a look at this image (Marty, 1999):



Its legend says:


  Seven basic pathways are used for the biogenesis, maintenance, and supplying of vacuoles. Pathway 1: entry and transport in the early secretory pathway (from ER to late Golgi compartments). Pathway 2: sorting of vacuolar proteins in the trans-Golgi network (TGN) to a pre/provacuolar compartment (PVC) via an early biosynthetic vacuolar pathway. Pathway 3: transport from PVC to vacuole via the late biosynthetic vacuolar pathway. Pathway 4: transport from early secretory steps (ER to Golgi complex; pathway 1) to the vacuole via an alternative route with possible material accretion from Golgi (indicated by the asterisk). Pathway 5: endocytotic pathway from the cell surface to the vacuole via endosomes. Pathway 6: cytoplasm to vacuole through autophagy by degradative or biosynthetic pathways. Pathway 7: transport of ions and solutes across the tonoplast. AV, autophagic vacuole; E, early endosome; ER, endoplasmic reticulum; PVC, pre/provacuolar compartment; TGN, trans-Golgi network.


According to the same paper (Marty, 1999):


  Experimental evidence suggests that material within the vacuolar system in plants derives confluently from both an intracellular biosynthetic pathway and a coordinated endocytotic pathway. (emphases mine)




Source: Marty, F. (1999). Plant Vacuoles. THE PLANT CELL ONLINE, 11(4), pp.587-600.