Ice cream is essentially a foam of ice crystals surrounded by sugar syrup, and air.
The amount of air--called overrun--introduced during the churning and freezing process controls how dense, heavy and "creamy" the ice cream is. The more air ice cream has, the easier it is to scoop, since there is less solid mass being cut through.
Ice cream with less overrun will be harder to scoop but have a smoother, heavier mouth feel. Hagen Dazs is an example of a supermarket ice cream with low overrun. Ice cream with more overrun--Breyer's is an example of a supermarket brand with the maximum legal 100% overrun--will have a lighter, fluffier mouth feel.
The second factor is temperature. Sugar syrup does not freeze at 32 degrees. As ice cream freezes, water crystals crystalize out of the syrup phase, making the syrup even more concentrated, and further lowering its freezing point.
The colder the ice cream is stored, the harder it will be simply because there will be more ice and less liquid syrup. Good commercial ice cream shops will store ice cream in the back freezer at -10 F, but warm it to 25 F in the counter freezers for service.
Lastly, if the scooping instrument is hot (for example, dipping the ice cream scoop in hot water), it will melt a bit as it cuts through. Sharper scoops are also more effective.
So your options for easier to scoop ice cream are:
- Serve it warmer, such as by "thawing" a bit in the refrigerator before serving.
- Buy or make ice cream with more overrun.
- Use a hot, sharp scoop.
- A bonus one--ice creams without hard chunks mixed in are usually easier to scoop.
Vodka or other ingredients in the mix will change the the freezing chemistry, but not radically--they are more for flavor (some flavorants dissolve in alcholol but not water, or because of the flavor in the liqueur itself).