Pick the challenge to enter it — that grants Challenger access, which is all a challenge needs. Vanguard is the separate track for CertiK's paid bug bounty programs: you submit vulnerability reports on live protocols and are paid for valid findings. It takes longer to review and you do not need it for the challenge.
Your standing on other bug-bounty platforms (e.g. Immunefi/Cantina/Code4rena leaderboard, rank, or notable results).