Option 1 is to use display:table-cell. You need to unfloat the Bootstrap col-* using float:none..
.center {
display:table-cell;
vertical-align:middle;
float:none;
}
Option 2 is display:flex to vertical align the row with flexbox:
.row.center {
display: flex;
align-items: center;
}
http://www.bootply.com/7rAuLpMCwr
Vertical centering is very different in Bootstrap 4. See this answer for Bootstrap 4 https://stackoverflow.com/a/41464397/171456