If you don't want the expression to be evaluated on the load, you can replace the ampersand with the Chr() function equavilent and the resulting variable will contain the ampersand.
Let vDim = Chr(39) & 'Dimension_' & Chr(39) & Chr(38) & 'Axis';
The Chr(39)s will put single quotes around Dimension_ and the Chr(38) will add the ampersand between them. The result of that expression should be:
You could also force spaces around the ampersand, but I don't think it is necessary.