Ok so to start with the legal vows. You will have to repeat, after the registrar, the legal vows, which is about a sentence's worth of words. Saying these vows is what makes you actually married, so you can't alter those. If you wish (you don't have to), you can add personal vows, which usually come after the legal bit. DISCLAIMER- check with your registrar. Some may not allow this, depending on the type of wedding you are having.
Having heard approx. 70 billion personal vows, these are my (private) thoughts on adding in personal vows.
Make it short and sweet. The time for eloquent, long, meandering witterings is after the ceremony, during the speeches. You have a limited time that your money buys for a ceremony (pre interviews, ceremony, readings, signing of the marriage schedule, witness signing, photographs), don't waste it with someone yakking on about how they met their one true beloved. Save that for the dinner speeches.
If you are going to have personal vows, write them down on a bit of card. When people go 'off piste' and try to speak from the heart, nerves get the better of them and it turns into a meandering soliloquy that people start shuffling and coughing through.
Tell the groom to have tissues in his pocket- I don't know why men never think of this but they never do. If the bride uses tissues, think about where are you going to put the snotty tissues afterwards. The registrar doesn't want them. Can you give them to someone else to put in a pocket?
I personally don't love personal vows that seem....sexist. I know it's not my business, and that it's between the couple getting married, but I cringe when I hear 'I promise not to nag you about football and always make you a protein smoothie and let you have a lie in on Sundays'. Also, and trust me on this, people will judge you on the vows you make. So you make think you are making a cute 'in-joke' but (like a couple I recently had) when the groom says 'I promise not to go mad when you use my credit card to go shopping' and she says 'I promise to give you a home cooked meal every day', I feel like it says quite a lot about that relationship. Weirdly, I've seen a trend where couples air their arguments in the vows, like 'I promise not to nag you about leaving your rugby kit on the bathroom floor for a week'. Don't do that. Don't promise what you won't do, state what you will do.
The best vows are the short, sweet and from the heart ones. Also PRACTICE saying them out loud and time yourself doing it. If they are more than a couple of minutes long, shorten them. I had a wedding recently where the groom pulled out 4 pieces of typed A4 paper of personal vows and the other registrar and I were like.....'oh lord, that's really going to mess with the timings of this wedding'. Then it turned out he wanted to translate them into another language and repeat them all over again. That is something that should have been saved for dinner speeches. I asked him how long he thought it would take to say and he clearly hadn't practised as said 'oh, um, 5 mins'. NO. 20 minutes later he was still banging on. We had to cut some of the ceremony and a reading to make sure they were actually legally married in the time allotted.
Tbh, you don't NEED personal vows. But if you want them, I'd say less than 2 or 3 mins, something from the heart, nothing too 'revealing' (like the bride that promised steak and blow job saturday, ugh), something that you will mean, and something that won't be held over your head in the future ('you promised me a protein smoothie every morning'). Side bar, one groom once said he would promise not to make fun of her belly unless she put on more weight. Bride looked like she was going to cry. But they'd already done the legal bits so technically were already married- too late to back out now.
Some couples share their vows with each other ahead of time, others don't. What I have noticed is that the couples that don't communicate it ahead of time have wildly differing lengths of vows as a result. Like one person has written two sides of A4 and the other has written a paragraph, and then the length of time it takes to say, and the difference between that, sort of hangs in the air like a 'ohh....you weren't in sync and now it's a bit awkward that everyone's noticed'.
If you get really stuck you can google suggested personal vows and tweak them. If you want to keep it safe, just do the legal vows and write something nice for after the ceremony that can be more in depth. If you do want to include something personal, have a practice, write them well in advance, and remember that it's not just your partner who will be hearing them.
Honestly, steak and blow job saturday scarred me a bit. Watching the guests faces' as she said that, I wanted to drop through the floor. And it was recorded, so that shit is on tape forever. Awks.