I have recipe for a popular biscuit mix substitute that calls for lard.
Can I substitute liquid veg oil in a biscuit recipe?

I wouldn't use liquid vegetable oil but would suggest using vegetable shortening. I make up a homemade bisquik mix and vegetable shortening works perfectly and the mix stores very well.
If I use a homemade mix, such as for pancakes that I whip up on the spot, I will add vegetable oil for something like pancakes but will cut in butter or shortening for biscuits, More of a flaky crumb. The shortening or butter when baking is what helps to create the air pockets you are looking for. Vegetable oil will not do that.

I also use vegetable shortening when I make my mix. I still use lard for some things - just a habit.

Lard is so much better.

There are biscuit recipes using oils... but one reason why those that use solid fats- such as lard or shortening- or even butter are preferred is the flakiness and tender bite that results from the tiny bits of solid fat being trapped between the multi layers of flour. They then "explode" when introduced to heat in a hot oven and the steam created as the solid fat melts causes the layers of flour to expand, creating the flakiness. With oil- the liquid fat is so thoroughly distributed that you don't see this effect in such a pronunced way. This is also the same set-up w/ pie pastry.

It can kinda be compared to the difference of sprinkling chopped or minced onion onto a hotdog- versus say puree'd onion that has been blended to a spreadable liquidy pulp. Same thing technically- yet the end result is entirely different, and makes for a similar- yet different taste all together.