To achieve parallelism you don't have to split uncompressed CSV file.
Using the Parallel Load Library
You can use the HP Vertica PloadDelimitedSource library with the COPY statement to parallel load delimited files. This section refers to the functionality as Pload.
The Pload feature is ideal for loading very large data files (at least 10 GB). The library divides file parsing tasks across each core on the server node where the data file resides, significantly reducing file load time. For example, given a 12GB file to load, performance could be 3 - 5 times faster than loading the file without Pload.
You can specify the file division size (in bytes) by supplying an integer value to the pload chunk_size parameter.
After installing the library, you can use the COPY statement's WITH SOURCE PloadDelimitedSource parameter. Using Pload with COPY LOCAL is not supported.