Can an igneous rock have a Vp of less than 1000 m/s and need blasting to excavate? I have run a 60 m refraction seismic line on a mountain summit. The environment was noisy (wind would be the principal agent) and quite difficult to pick first arrivals. It resulted in 800 m/s.

A borehole has shown the profile stands the same after 0.5 m depth of biologic weathering with no fault nor joints to 10 m. Sample taken only at 1 m shows UCS 5 MPa, 10% water absorption which leads to 0.16 porosity, 17.5 kN/m$^3$ bulk density. My geologist partner has identified it as rhyolite or andesite, and attributes the low density to weathering action that can extend to several meters.

A backhoe couldn't excavate more than 1.0 m. So my concern of rippability expected based on Seismic velocity Vp.

Rock type has been estimated visually and according to a general geologic map (I'm sure it is an igneous rock. It could be also ignimbrite) . All tests results has been shown above, and I'm confident about them.

Can an igneous rock have a Vp of less than 1000 m/s on this situation and need blasting?
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.