If I encounter an error during the import process, do I need to import the whole data again?

The answer is NO.

Since SourceAnywhere Standalone 2.3.1, the performance of VSS Import Tool has been significantly improved.

The import process is now divided into four stages: import items, import file content, import pin status and import labels.

If an error occurs after the items import stage, you'll just need to restart the import tool. It will automatically start from the error point rather than from the very beginning.

To learn more about what's new in VSS Import Tool, please refer to:
http://www.kevingao.net/sourcesafe/new-import-vss-to-sourceanywhere-tool.html