Vegas marriages are equally valid as a California marriage. The requirements for a legal marriage in either State are basically the same: a marriage license, a pronouncement of marriage by someone legally authorized to do so, and usually at least one witness. A marriage in one State will be recognized in any other State as long as it was valid in the State where it was performed (except possibly a same-sex marriage).
Prenuptial agreements are different. If you want a prenuptial, and you want it to be enforceable, then you better contact a very knowledgeable and qualified family law attorney long before your expected wedding.
If you already got married, you can also enter into a postnuptial agreement, but again, you will need an attorney for that.
Whether it is pre or post, your spouse will also need his or her own attorney.
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