Assuming you're taking a job for the long-haul and not just a locums position, then I think taking 1-3months off before starting would not be unusual.
By the time you're down to negotiating start date the employer has very little bargaining power left - they've offer you the job, and probably don't want to take several months trying to find someone else. Of course they will want you to start earlier, but this is all part of the bargaining process.
I thought psychation was vacation, so isn't it pointless to take vacation before taking on a position in psychation. Furthermore, you won't get paid if you take vacation if you are not already on psychation.