Seriously, in my experience there really is no way to reheat dry-roasted meat (not just beef) and achieve an acceptable result. Both flavor and texture change when you reheat. It is the unsaturated fatty acids which are damaged when exposed to oxygen. It starts the minute you put it in the fridge (or open the vacuum seal), and then speeds up when you reheat.

Putting it in a very low oven works but takes a long time, up to 40 minutes a pound; at 225. Use a meat thermometer, aim for 125.

If you are willing to pre-slice, an old catering trick used especially for thick slices of prime rib which have been pre-cooked: line a baking sheet with leaves of iceberg lettuce, top with beef in a single layer and cover with more lettuce. Reheat in medium oven. The moisture from the lettuce will keep the beef from losing its red or pink color.

If reheating whole, have at room temp for at least an hour before reheating.

This is what the Texas Aggies suggest for whole pre-cooked:

"Remove from bag and wrap in foil. Place in a 300° F oven and monitor with a meat thermometer until desired internal temperature is reached. Recommended internal temperature is 135° F. Approximate cook time is 2.5 – 3.5 hours. Cook time will vary based on weight and oven temperature. Remember you are just reheating, the roast is cooked. Don’t over cook."