"The names don’t match up, because on Microsoft’s Marketplace the author handle (display name) is used, but Open VSX returns the username as the author name."
I know it's insignificant, but that seems like a relatively easy and (marginally?) useful thing to change ;)
This isn't a huge issue, but it's one of the larger issues why FOSS alternatives are not often as well received as their proprietary counterparts (see: unix). Consumers/users should be the first/only consideration when designing user-facing software, but it's a rare sight to see in FOSS. I wish more FOSS developers cared about the software instead of the code, because the difference manifests in decisions like this.
If displaying the same "author" data resulted in users assuming that a given username on the Open VSX site should be trusted just because it happened to match a username on the VS Code Marketplace, it's probably a good thing that Open VSX displays the name differently. These are two different sites, and two different accounts; trust in one should not imply trust in the other. It would be even better if Open VSX could somehow ensure that the displayed author names never match the corresponding projects on VS Code Marketplace, for example by integrating a domain name or other globally-unique component into the author field.
Such a scheme could then enforce policy in the verification process that imposes restrictions on what the display name would be allowed to be.