vscode-oss.desktop defines the icon for VS Code to be "code-oss", which does not seem to be included in the majority of icon packs. Instead, most icon packs only include a "visual-stuido-code" icon.
Whilst having a different icon for different versions of VS Code may be desirable, the missing icon can cause an inconsistent look when using an icon pack which does not have a specific icon for the OSS version of VS Code (I'm not sure that the default theme in Solus even includes one?).
As I understand it, both the MS and OSS versions of VS Code used to be packaged for Solus, however it seems that the MS version has been removed and the OSS version is the only version available. As such, one possible solution may be to simply change the Icon for the OSS version to "visual-studio-code" icon which is much more common.