I'm not sure how current this is, but Vell's Heart should be about a 1% chance. It'll also sell for a decent price, definetely a good way to upgrade your gear. I'd say ultimately, it's up to you to decide. Especially if you like to have fun in your own way, this would really just increase your AP along with some other benefits.

Personally, if you're coming back to BDO and don't have at least full TET Boss Gear, I'd make a new character and play through the Season. After you graduate, you'll have a nice bonus AP available.

The season is a great way to catch-up to the average gear, you can also go through the Magnus questline to get another quick upgrade in Boss Gear.

I personally would keep it, its going to be difficult to reobtain that item and a lot of people use it even at end game.

The rule of thumb is, keep the first of what you get, and sell anything that you have excess of. You essentially obtained the best in slot item (in most cases). In general, it's not easy to obtain vell's heart. Ordinarly there is a backorder so you have to compete against others to obtain one but during events like these, that may not be the case in your region. I would highly recommend that you keep it since money's end goal is so you can afford best in slot equipment and well, you just obtained it. Should you be fortunate enough to obtain another in the future, you can sell it and who knows if they ever plan on increasing the price of them in the future.