<input id="_535" type="checkbox" onclick="ChangeCheckBox(this)" value="535" name="vac[kls_type][]"/> <label id="label_535" for="_535">Маркетинг</label>
alert(document.getElementById('label_535').innerHTML);
<input id="_535" type="checkbox" onclick="ChangeCheckBox(this)" value="535" name="vac[kls_type][]"/><label for="_535">Маркетинг</label>
alert(document.getelementById("_535").nextSibling)