@Murcielago Thanks again for your suggestions. I tried exporting from the Proton web app, but still no luck getting the GrapheneOS Contacts app to import them. As per my earlier post Fossify Contacts has no issues importing the file and I also tested it in Outlook with no problems. Not sure what the issue is with GrapheneOS.
I didn't want to have to download Fossify Phone and Messaging apps, so went looking for a solution to the issue of the GrapheneOS apps not being able to see them and found a workaround in this bug report: https://github.com/FossifyOrg/Contacts/issues/225
You need to create a dummy contact in the GrapheneOS Contacts app before importing the vcf file into Fossify Contacts in order for the "Phone storage" option to be available in the dialog instead of just "Phone storage (not visible by other apps)". If you select the "Phone storage" option, the contacts will be visible to all the apps with contacts permission. I called my dummy contact "Zzz" so it's on the bottom of the list.
I thought about uninstalling Fossify but noticed it displays birthdates that GrapheneOS doesn't, so decided to keep both. It's a bit of a kludge, but it works. :)