JSFiddle - React, Tailwind, and code Playground

HTML

<select name="b_company" id="b_company">
    <option value="0" selected="selected">Individual</option>
    <option value="1">Company</option>
</select>
<div class="control-group" id="vat" style="display:none;">
    <label class="control-label" for="vatnumber">V.A.T. Number</label>
    <div class="controls">
        <input id="s_vatnumber" type="text" name="s_vatnumber" value="" />
    </div>
</div>

JavaScript

window.onload = function () {
    document.getElementById('b_company').addEventListener('change', function () {
        if (this.value == 1) {
            document.getElementById('vat').style.display = 'block';
        } else {
            document.getElementById('vat').style.display = 'none';
        }
    }, false)
};