Can an igneous rock have a Vp of less than 1000 m/s and need blasting to excavate?
That seems way too low, even for crustal rocks. Recall: Vp = $\sqrt{[(K + 4G/3)/]}$

If you have a rough idea of the mineralogy of the rock (you do if you know that it's andesite or rhyolite), you can estimate the bulk modulus (K) and shear modulus (G) with a Voigt-Reuss-Hill average and use a reasonable density value (maybe 2500 kg/m3) to get a reasonable number for Vp.

This Wikipedia page has a table of velocities for common rocks that might be helpful.