Why does the plugin create more than 1 sitemap?
Video Sitemap Pro plugin creates separate sitemap files in two scenarios
Video Sitemap Pro splits sitemaps per 1000 URL.This means every sitemap contains at max 1,000 URL.So if your website has more than 1,000 URL two sitemaps will be created.This ensures your sitemap file size is less and is easy on Google bots.
Video Sitemap Pro creates separate sitemap files for different video providers supported.For example if you are using YouTube and Vimeo then two sitemap files will be created.One for YouTube and other for Vimeo.
The index sitemap file includes all the regular sitemap files created.You should add this index sitemap file to Google.
Which sitemap map should be added to Google?
You should always add the index sitemap file for example your-domain.com/sitemap-index-video.xml
Can I use your plugin with regular sitemap plugin?
Yes you can this plugin with regular sitemap plugin.Video sitemap pro deal with videos only.So both sitemap files play well together. Here is link stating Google point of view on duplicate URL in sitemaps
File is not writable error
If you are getting file not writable error then make sure your web root is writable. You can create below blank files and make them writable (664 file permission)