Align one div inside another vertically

Question

My goal is to get this vertical alignment:

I need to align vertically and horizontally the contents of div's busca and buscaImovel consecutively.

Horizontal alignment works but vertical alignment does not work.

What am I doing wrong?

Obs.: I tried to put position: relative; in the search div but both search and searchImovel disappeared.

CSS

.busca {
    top:370px;
    height:240px;
    background-image:url(../../_img/fundo_ap.jpg);
    background-repeat:no-repeat;
    background-size:cover;
}


 .buscaImovel {
        position:relative;
        top:20px;
        width:100%;
        height:200px;
        background-color:rgba(0,0,0,0.6);
        text-align:center;
        -webkit-border-radius: 20px 20px 20px 20px;
        -moz-border-radius:  20px 20px 20px 20px;
        border-radius:  20px 20px 20px 20px;
    }

    .campoBusca {
        position:relative;  
        top:20px;
    }
    .camposBusca, .alternativaEscolha {
        display:inline-block;
    }

HTML

<div class="busca">
  <div class="buscaImovel"><br />
    <h1 >Pesquise pelo imóvel que deseja</h1>
    <form action="busca.php"  method="post">
      <div class="camposBusca">
         <label class="labelPequeno">Tipo</label><br />
         <select class="typeTextMedio" id="tipo" name="tipo" required>
           <option value="">Escolha o Tipo</option>
         </select>
      </div>

      <div class="camposBusca">
         <label class="labelPequeno">Bairro</label><br />
         <select class="typeTextMedio" id="bairro" name="bairro" required>
           <option value="Indiferente">Indiferente</option>
         </select>
      </div>

      <div class="camposBusca">    
         <button>Buscar</button>
      </div>

    </form>

  <div style="height:15px;"></div>

    <div class="alternativaEscolha">
       <a href="buscaAvancada.php"><label class="labelGrande" style="background-color:#CCC;">Busca Avançada</label></a>
    </div>

    <div class="alternativaEscolha">
    <!--abrir popUp-->
       <a href="buscaCodigo.php"><label class="labelGrande" style="background-color:#CCC;">Busca por Código</label></a>
    </div>
 </div>
</div>

As it stands, the content of div busca, when on a screen of 320px X 480px, surpasses the lower div and becomes a mess