How to align buttons on TwinColSelect vertically?

I’m using Vaadin 6.8.5 and I have problem with TwinColSelect (see
Vaadin Sampler
).

I want to align two buttons vertically to middle. They are on the top now. I tried CSS with no efect.

.v-select-twincol-buttons {
    display: inline-block;
    vertical-align: middle;
}

Workaround can be also buttons expanded to entire empty space between left and right column.

Can someone please help me with this?

not a pretty solution but works.

assuming the twin column has height about 150px,

.v-select-twincol-buttons {
height: 150px;
padding-top: 40px;
}

you can change the height and padding top based on the size of twin column select.