Vue.js comes with a lot of features that make working with forms easier out of the box, like the reactivity and events with modifiers and the two-way binding system on top of it. But if the libraries can speed up the development process and provide a ready to use, well tested environment, it may be worth to just use them instead of baking own alternative.
# Summary (TLDR)
Here are the top pics from each category
Vee-Validate - little boilerplate, very intuitive, vastly popular
Vuelidate - very customisable
Vue Form Generator - provides the most fields out of the box
Vue Formulate - covers both form generator and state management use cases
# Form Validation
You can write a simple form validation yourself, but if you want a well tested solution that's easy to get help with from other Vue developers, give a try to one of the libraries described below.
Vee-Validate is a an all-inclusive library with string-based validators. One of its biggest advantages is that it's fully compatible with form validation of Laravel framework, which made it very popular among PHP developers. The validation is applied to form fields using scoped slots, allowing the users to keep the validators directly in the template if such is their preference. Another advantage is that it offers localized messages for validation rules.
With Vuelidate you don't validate your form fields directly, but rather by validating the data of your component, which state is compared to provided validation schema. Unlike string-based Vee-Validate, all validators in Vuelidate are functions. Vuelidate has many useful features but you will have to write some additional code too or use 3rd party plugins, like for the missing error messages handling.
# Form Generators
Form generators are libraries that allow users to generate a set of form elements based on so called schema - data representation of form fields with support for various custom properties that are later applied to some more or less generic templates. There were multiple projects with an ambition to write an ideal form generator for Vue.js, but out of them, the following ones achieved a state in which they're suitable for actual use.
Vue-Form-Generator is the most popular form generator for Vue.js. It uses an array based schema, it's own validation solution and a set of templates based on Bootstrap. You can use one of two builds - basic with most commonly used fields and the full one, with all the bells and whistles the library has to offer. Both can be extended with your custom fields.
The library has good documentation, high test coverage, and was the first fully production ready project of its kind for Vue.
# Vue Formulate Most Flexible
Vue Formulate excels in its flexibility. You can treat it as a form generator or compose the forms directly in the template from provided components. Out of the box, it comes without the more complex components available for Vue-Form-Generator, but it's easy to create your own thanks to robust plugin and scoped slots architecture, which gives hopes for an ecosystem of 3rd party extensions in the future. You're also not bound to a specific CSS framework.
The project is well maintained and offers extensive documentation, good internalization support, and built-in validators.
It allows you to write your custom set of templates or use the standard components for Bootstrap, which are available as a separate library. Just like Vue-Form-Generator, it uses its own validation system.
# Form State Management
Form state management libraries usually have smaller scope than form generators and their main task is to help the developer to get rid of the common boilerplate. It takes care of the most annoying parts of writing forms, while still leaving you the full control over your templates. Such libraries can have different structure, based on classes, scoped slots or mixins.
Vue-Final-Form is a scoped slots based integration of Final Form, a framework-agnostic library that became pretty famous thanks to its subscription based state management system. Among its advantages are nice API and the focus on performance. Being written by Egoist, a Vue.js Core Team alumni, will also be a plus for many.
Vue-SimpleForm is heavily influenced by Formik, a popular forms solution in React ecosystem. In order to use it, you just pass the form with its fields as a scoped slot to a wrapper component and it manages the state for you. If you're familiar with Formik because of your React experience and like how it works, by all means give it a go.
# Other Libraries
There are many kinds of form related libraries that serve different purposes. Let's mention the most popular among them.
Vue-Form-Wizard allows you to create a multiple step form wizard, commonly used in online shops and other commercial web apps.
You can use it with UI and validation libraries of your choice, it also provides support for Vue-Router. Its author, Cristi Jora, took the effort to provide comprehensive documentation and many code examples for different use cases.
Out of all kinds of form fields, multiselect is usually the most tricky to implement. Luckily Damian Dulisz from the Core Team took care of that and wrote Vue-Multiselect. Thanks to smart architecture, it's highly configurable and you can make it look the way it fully fits your design.
# Learning materials
Here's a list of selected general materials on how to write forms in Vue.js that don't require using any specific library.