Catch up boosts
From Path of the Vampire Wiki
Catch up boosts are a karma 'item' which can be purchased on the Karma Shop page. Catch up boosts allow you to regain up to 50% of wasted energy or PXP (due to vacation or similar). This should allow you to regain some ground lost during a hiatus. The amount you can buy depends entirely on how much energy you left unspent in the past couple of months, or how much PXP you didn't gain due to not feeding, etc.
PXP Catch Up
PXP orbs (Which give 10 pxp per) are one of these catch-up items. You still get to buy 3 PXP orb per month, regardless of how much PXP you didn't gain last month, but now you can buy more if you failed to gain PXP recently. Because you can only catch up 50% of what you lost, you must have missed out on at least 20 PXP in order to buy 10 PXP back. All of these figures are on the Karma Shop page itself.
Energy Catch Up
You can also buy back lost energy. Every hour when your new energy is calculated, the system remembers how many extraneous energy you had numbering more than you maximum energy cap. That total is ongoing, and when you reach more than 500 recently lost energy, you can start to buy energy boosts. Energy boosts give 50 energy points back, but you can only buy 1 for every 100 energy you missed out on. These cost 2 karma points per 50 energy.
At the start of every month, these ongoing totals of wasted energy and PXP are halved, making it so that those who have been inactive for a year can't simply return and level up 20 times, though they will be able to catch up a fair amount if they wish.