When in March? Semana Santa is at the end of the month.
Like you have read weather is totally unpredictable. Take something of everything.
Weather history web page may give you a clue but they only give averages. So expect something above or below that average.
Veg locations:
http://www.happycow.net
http://azahar-sevilla.com/sevilletapas/cuisine/vegetarian/

This is my absolute favorite go-to guide for Sevilla: http://maribelsguides.com/mg_sevilla.pdf
How long will you be there? There are plenty of sights "on" the beaten track that are amazing (the Alcazar, Cathedral, the whole Barrio Santa Cruz...) and if you're looking for a Flamenco show, absolutely check out La Casa de la Memoria. The best Flamenco show for sure!!
Weather is for sure unpredictable, but by no means hot in March. Even if it warms up in the afternoons, you'll at least need something for the evenings. Last time we were there it was the beginning of April and we did lots of pants/short sleeves/light coat (which got tossed back to the hotel after breakfast). Hope that helps a bit!

Take a look at the website www.ExploreSevilla.com. It will give you most of what you are looking for.

Thank you so much for the info and the links.
We are going on the 16th for 1 week staying for 5 nights in Seville and 2 nights in Ronda.
We will have a hire car so can get out and about but it sounds as if we will never see everything in one week

Bummer, you will just miss Semana Santa! An amazing experience, but you'll still have a lovely time for sure! Are you staying close to the center? Travel within Sevilla is absolutely doable without a car (and in some cases having a car may be more of a pain...any way to ditch it?). I think 5 days is a wonderful amount of time, it's such a beautiful city!

The exploreseville website is probabably my favorite for all around details on Sevilla. I'll be there the same time as you this year and visit around Holy Week every year. Weather as others have said is unpredictable. I've never had a March that I'd consider hot but have been there when it was in the 70's and quite nice. However, there have been visits where it has been high's in the 50's and rainy. Ronda may be a bit chilly in the mountains.

A great site for food in Sevilla is:

http://azahar-sevilla.com/sevilletapas/

I was there in March once for Holy Week and I think it was towards the end of March. I can't imagine going there in March thinking it is summertime. It was pleasant enough in the day time, but it rained several days (and nights) and it was probably down to at least 50 or mid-40s at night. I wore a jacket, boots and slacks going out at night (plus a pullover sweater). My jacket was more than a thin one, also, but you know what it is like when it is 45 degrees and can decide for yourself. Even in the day time, I never wore what you'd wear for summer clothes. For example, I am not sure I ever wore sandals and caprise, but if it were ever that warm, it usually wasn't and I wore some kind of walking shoes and jeans or long pants. I didn't wear a coat during the day time.

I think you can be checking the weather several weeks before you go to get an idea, but it can always change, of course.

In Europe we work in Centigrade these days!

As we do in the UK ribeirasacra!!!Fingers crossed for temps in the 15c plus

No problem wrapping up in the evening (will bring coat) but in the daytime it would be soooooo good to feel the sunshine on my face and sit outside a cafe for a coffee (or a beer at lunchtime)

we will pick our car up at the airport and then most likely leave it in the hotel carpark until we go to Ronda.

Thank you all for the info it has been most helpful and much appreciated.

After Seville our next trip will be in May to Majorca to see our son compete in the Half Iron Man!! All the family are going, we are Grandma & Grandad!!! so you can imagine with 4 grownup children, partners and grandchildren there are quite a few of us and luckily we have managed to find a beach house big enough for all of us!!

So as you can imagine we want everything in Seville to be perfect for just the 2 of us!!!