I often hear the word vertice (pronounced VER-tee-cee) used as the singular of vertices instead of vertex.
- Is vertice an acceptable singular for vertices? If so, how is it pronounced?
- On the other hand, is vertexes an acceptable plural for vertex?
The singular is vertex.
According to the Merriam-Webster dictionary entry for vertex, both vertices and vertexes can be used as the plural of vertex. Vertice is not listed as a word.
The same happens with index, for example, where the plural can be both indices and indexes.
According to the same dictionary, vertex and index both originated from Latin:
Explanation for the two forms of plural from World Wide Words:
Notice that based on the last paragraph above, the author of the article does not consider vertexes a valid plural of vertex.
Additionally, vertices is much more used than vertexes: 12.600.000 occurrences of vertices vs. 26.700 occurrences of vertexes in Google. My Firefox's spell checker disagrees though. It doesn't know vertices but it does know vertexes.
As for vertex vs. vertice, there are 2.680.000 occurrences of vertex in Google and 130.000 occurrences of vertice.
These numbers also lead to a possible explanation for why people sometimes say vertice. The plural form vertices is more than 4 times more used than the correct singular form vertex. So it's possible that people derive vertice from vertices because they hear the plural form vertices much more often than the singular form vertex.
Neither Merriam-Webster nor Wiktionary have an entry for vertice, and I have never encountered it myself. Then again, I am heavily biased, working a lot with graphs.
All that being said, I think it is only natural for someone who encounters the plural form vertices for the first time in his life to derive the singular form as vertice. Looking at your location and its demographics, I think that Spanish influence could be yet another factor in your immediate environment, the Spanish word for vertex being vértice.