If vesicles can fuse to membranes fairly easily, what’s there to stop a whole bunch of cells fusing together?
Nothing inherently stops it, and in fact there are types of organisms where that happens. For instance, there are two main categories of slime molds. All slime molds go through phases where they are single cells, and then they come together into a large multicellular Mass, but in some slime molds those cells actually lose their cell walls and become in essence a single huge cell.
The same thing happens in your own body too. Every one of your muscle fibers is a single cell, but they are multinucleated cells because they originated from hundreds or thousands of different cells that fuse together.
The reason it does not always happen is because there are a lot of disadvantages to becoming all the same cell as well as advantages. The rule of powers means that as a three-dimensional object like a cell gets larger, it has less surface per unit of volume. Since cells have to exchange all of their gases and nutrients across the membrane, once they get too large they can’t effectively do that anymore.
It can happen. In medicine the abnormal fusion of two internal organs is called a [fistula](https://en.m.wikipedia.org/wiki/Fistula), and they are generally caused by injury or disease. As the body tries to heal something it can somehow accidentally stick internal structures together. My wife is a surgical PA and it’s not totally unusual for them to find fistulas in people with a history of surgery or other internal complications.
Cell membranes are different from vesicles. They contain anti-fusing lipids like Lysolipids. Also, they have phospholipids that are negatively charged and repel each other.
Basically, it is energetically unfavourable for cell membrane fusion to occur and will not happen spontaneously. You need deliberate energy expenditure to merge cells, such as monocytes merging into macrophages.