In "nos premières vacances toute seules", why is it "toute" and not "toutes", to go with "seules"?
Freeform Writing Exercise B2
The reason why the adverb 'tout' has some interesting non-agreement rules in the case of feminine adjectives is explained in the following Kwiziq lesson which has just been updated to include this.
Also, take a look at the Q&A at the bottom of the lesson as this has been asked already.
It's interesting that you don't think it should also agree in number in this case. I have a feeling I've seen the rule defined that way somewhere before, although I don't remember where. However, all the references that I can find now say that it should also agree in number. For example:
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