In this situation, it’s unimportant that the secret you want to store is a certificate - it’s just opaque data as far as Vault is concerned. Vault can store arbitrary user data in JSON format in its KV secrets engine - KV - Secrets Engines | Vault | HashiCorp Developer
Mostly… it can store anything that is a valid JSON object meaning you can’t store raw binary data unless you apply your own encoding, e.g. base64, first, to make it fit within the restrictions placed on a JSON string.
Total size stored in any one KV entry is limited as well - the exact limit depends on the choice of storage backend used for Vault as a whole, and various internal overheads, but I estimate that more that 500 kiB would be cause for concern. This shouldn’t be an issue for certificates, which tend to be much smaller than this.