Hi JonG
In my experience on the Frances - most recently 2019 and 2022 - every albergue I have stayed at that offered a communal was able, and usually willing, to provide a vegetarian meal option. It may not always have been very fancy, but the important thing was that people weren't difficult about it, were usually very happy to try and help, and often were enthusiastically veggie or vegan friendly themselves (particularly volunteer hospitalera/os).
Communal meal options won't occur that frequently, so you are more likely to be making a meal in a communal kitchen or eating out. On the Frances you are usually in towns or villages with super or mini mercados that will have a sufficiently good range of foodstuffs for you to prepare a veggie dinner. And eating out will usually also give you some veggie options. Each establishment along the Way will have been asked if they have veggie/vegan meals thousands of times before you - and most will have realised it's worth adapting accordingly and got used to it. Just make sure you don't accidentally veer off the camino bubble as 'old Spain' lies just outside..