Hi J, you're off to an excellent start saving for your retirement. I'm assuming your employer's 401k ira is a traditional 401k plan (where you put money in pre-tax). If that's so, the Roth IRA makes sense for you, so that you will eventually have both types of money available in retirement: pre-tax now (401k) that will eventually be taxed when withdrawn in retirement; and after-tax (Roth IRA) that will be tax-free in retirement (assuming the money has been in the Roth for at least 5 years).
Since you can't make a direct Roth contribution, you can make an after-tax contribution to a traditional IRA, and then within a few days of the contribution money clearing your investment account, you can convert the after-tax IRA money to a Roth IRA (I'm also assuming you have no other IRA money right now; if not, the conversion process is a little more complicated and will likely need the help of a good tax advisor).
Low-cost index funds (like those offered by Vanguard) are an excellent choice for all your retirement money (401k, IRAs, etc.). At some point, it may well be worth your while to spend some money hiring a professional investment advisor to help you with your investment choices. Good luck!
You are clearly on the road to financial independence. Rule number one, save some of what you make for the long run. I agree with at least putting as much as your employer will match into your 401(k) plan. It’s one of the best returns you can get for your investment dollar. I would recommend that you find a financial professional to work for you. You should develop a comprehensive financial plan. You will gain better knowledge of which investment vehicles will work best for you. Interview a few candidates and hire the professional that jells with you the best. People don’t plan to fail. Unfortunately some people fail to plan.
I applaud your decision to save! Nice work! My suggestion is to max your 401k contribution first. You get the multiple benefits of lowering your AGI for income taxes, getting an employer match, and growing those assets tax-deferred. And I think that saving into an IRA - whether it be regular or Roth - for current tax benefit or not - is great. But make sure that you first dedicate some savings - say 6 months of living expenses - to an emergency savings fund. Should you become unemployed, you don't want to have to tap your retirement accounts and pay a penalty. Keep up the good work!