JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdn.jsdelivr.net/vue/latest/vue.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">

<div id="app">
                <div class="container">
                    <button class="btn btn-success mt-5 mb-5" @click="addNewSection">
                        New Deal Section
                    </button>

                    <div class="card mb-3" v-for="(section, index) in sections">
                        <hr>
                        <div class="card-body">
                            <button class="btn btn-success mt-5 mb-5" @click="addNewItem(index)"> <!-- passing the index -->
                                New Item
                            </button>

                            <span class="float-right" style="cursor:pointer" @click="deleteSection(index)">
                                X
                            </span>

                            <h4 class="card-title">Deal section {{ index + 1}}</h4>

                            <div class="employee-form">
                                <input type="text" class="form-control mb-2" placeholder="Item" v-model="section.item">
                            </div>

                            <div class="employee-form" v-for="(addition, indexSection) in section.additionals"> <!-- additionals of the section -->

                                <input type="text" class="form-control mb-2" placeholder="Addition" v-model="addition.item">

                                <span class="float-right" style="cursor:pointer" @click="deleteAdditional(index,indexSection)">
                                    X
                                </span>
                            </div>
                        </div>
                    </div>
                </div>
            </div>

JavaScript

var app = new Vue({
                    el: '.container',
                    data: {
                        sections: [
                            {
                                item: '',
                                additionals: [] // <-
                            }
                        ]

                    },
                    methods: {
                        addNewSection() {
                            this.sections.push({
                                item: '',
                                additionals: [] // <-
                            })
                        },
                        addNewItem(index) {
                            this.sections[index].additionals.push({
                                item: ''
                            })
                        },
                        deleteSection(index) {
                            this.sections.splice(index,1)
                        },
                        deleteAdditional(index,indexSection) {
                            //this.sections[index].additionals.splice(index,1)
                            this.sections[index].additionals[indexSection].splice(indexSection,1)
                        }
                    }
                })