I got this job at this new company. I used my legal name when I filled out the job application form. I also had my legal name on my resume as well when I applied for this job.

Now, I just got the welcome email from the company stating my starting date and other information. They said, "Feel free to ask any questions". Is this a good opportunity for me to notify them that I would like to be called "Vin" and not "Shervin" around the office? Ideally, I would like my preferred name to appear in my company email, Slack username, JIRA, etc. I think it would be less confusing for my co-workers.

Should I notify them via email?

Yes, this is the right time.

At my company, people are asked this at the end of the interview process ("do you have a preferred name?"). So, we have plenty of names like Dave, Mike, Bev, etc.

I use a shortened name (ostensibly to be slightly different from an existing team member), and my shorter name is now all over the employee databases.

If the company has to use your formal name, they'll tell you.

But, let them know that you want to use a more informal form of your name sooner rather than later.

It is the best time for such a request, because they are now starting the process of adding you to all kinds of databases, creating accounts for you, updating door signs, printing name tags and what not. You give them the chance to "get it right" from the very beginning.

If you wait until your first day of work, most of this work could be done already. Informing them then will either result in a lot of rolled eyes and additional work for some poor lad or in your request being ignored.

Yes. Do it at the start.

My legal name is Edward but I am known as Ed.

The only person to call me Edward is my mother, and that is an indication that I have done something wrong.

Other times she calls me Ed.

Do it as soon as possible.

When I signed my contract with my current employer I asked it first thing.

Even though I got a 'no' as an answer that was because they use heavily interconnected systems (including payroll).

If there is no practical reason for denying it, your request should be easy.

Make sure you contact them as soon as possible, once you have your first name in your email address you can never get rid of it (talking from experience).

Definitely do this as soon as possible.

One place I worked was sold to a larger company, and that company added everyone to their IT systems under the name taken from their passports. I think about 15% of employees appeared with slightly different to totally different names. Turned out to be impossible to change.

(Reading Ed Heal's answer: We also had an Ed who got changed to Edward. He was not happy.).

Yep, bring it up now. If they set you up name tag or ID, set you up an e-mail address or other logins or accounts for company systems, they'll typically be only too happy to use your preferred name now rather than changing it later. That said, everywhere I've worked coworkers tend to pick up each other's nicknames pretty fast anyway.