I created v-form like this
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Couldn't comment on the accepted solution since I'm new to StackOverflow and wanted to provide my solution to this. I took the same initial step to investigate as OP and did a console.log(this.$ref.form), the output on the console is actually an array of [VueComponent] and validate() function doesn't exist in that context.
I was able to access the validate() function on the form by doing this.$ref.form[0].validate()