Honesty
Your word is your bond. Never break a solemn vow. Keep your tongue honest when dealing with those who have been fair to you and yours. Never speak a lie to a fellow follower of Mithras.
Honesty is simultaneously perhaps the simplest of the Mithraic values, and one of the most difficult to actually hold to with complete firmness, especially as a vampire. On the surface, don't lie seems simple and uncontroversial enough - but as a supernatural living in a world where the supernatural is a secret from the vast majority of the population for good reason, not to mention a vampire dealing with vampiric society and all of its politics and deceptions, it gets very difficult very fast. For that matter, we often don't think about little "white lies" we tell every day, little deceptions we uphold for politeness's sake - pretending to get along with a coworker who actually rubs you the wrong way with every other sentence, nodding along to someone's opinions you disagree with because you don't have the energy to fight them on it (or it wouldn't be safe to do so), things like that. Technically speaking, a total ban on deception bans these as well. How far you push that ban on deception depends on the individual - in theory, it should be total, at least between Mithraics, but in practice, practicality does win out in some of these "little white lie" circumstances (though most of us try to avoid even that as much as possible, and instead to foster a culture where stating the uncomfortable opinion as kindly and politely as possible isn't as offensive as it is in mainstream Western culture).
It should also be noted that, as with much of its social contract, Mithraism also tends to consider the ban on deception to only apply with other Mithraics. When it comes to kine, this is largely a matter of practicality - it's just not safe to be completely open and honest with kine these nights, and Mithraics recognize that the same as everyone else. When safety and honesty conflict, safety does generally rule. And when it comes to Kindred... well, there's the matter of the Betrayal, and the fact that many Mithraics consider other Kindred to be untrustworthy by default, releasing us from our obligation to be honest with them. In my short summary at the top of this section, I chose a very specific phrase - when dealing with those who have been fair to you and yours. This is my personal philosophy on the matter, and not universal; I tend to take an "innocent until proven guilty" approach with non-Mithraic Kindred, while others may take a more cautious approach and remain wary of outsiders until they prove themselves trustworthy, feeling that given how widespread deceit and betrayal are in vampiric society, it's reasonable to do so. Debates on the most correct approach to this run long; personally, I don't find it productive to assume the worst of people right off the bat. I don't necessarily hold myself to as strict a policy of honesty with non-Mithraic strangers as I do fellow Mithraics, but I do try to be as honest as possible.
Justice
In all things act with fairness, and mete out reward and punishment in accordance with the actions that earn them. Actions lead to consequences, both positive and negative. It is just to reward the faithful, the honorable, and the good. It is just to punish the liar, the traitor, and the thief. Act with fairness and obey the laws laid down by teaching, by temple, and by priest.
The Mithraic concept of justice is not always gentle, but it does strive to be fair. Mithraism does believe in the "sword of justice," that punitive justice is sometimes necessary and fair; some debate this, arguing that moving mostly or entirely toward rehabilitative justice would be more just, but others point out the unfortunate fact that in some cases vampires are just too difficult to contain long-term for rehabilitative justice to be possible. (Much less other supernaturals. Have you ever tried to keep a mage in a box they don't want to be in?) The debate goes back and forth, but in my experience at least usually lands on penance that involves repairing whatever harm was done if that's possible, and making up for it in some way if not (money paid, jobs done, or boons sworn to the wronged party are common). In extreme cases, that may not be enough - murder of a Mithraic, for example, warrants the death penalty unless there are extreme extenuating circumstances.
Author's Note: I am certain there is something significant I'm missing here about how all this works, but I can't figure out what it is or even what gap it would fit into, and I feel weirder the longer I sit here staring at it trying to figure it out, so I'm letting it lie for now.
Guardianship
Protect those within your care as the shepherd guards the sheep. Protect those you have the ability to protect; accept the protection of those stronger than you. Guide those you have the ability to guide; heed the guidance of those wiser than you.
Mithraism has a strong emphasis on community, being able to trust and cooperate with each other, and part of that is a responsibility for taking care of each other. We uplift each other, we protect each other, and we help each other wherever we can. A Mithraic vampire is by nature more powerful than a Mithraic mortal or ghoul, and as such it's our responsibility to protect them from forces they can't defend themselves adequately against - primarily, other vampires (and sometimes werewolves). Similarly, a thousand-year-old Mithraic has had a lot more life experience than a thirty-year-old one, and it's their duty to their community to share the knowledge and wisdom they've accumulated over that time. But it's not always a dynamic that goes in the direction you'd expect - on the flipside, the thirty-year-old Mithraic probably understands modern technology and modern kine society a lot better than the methuselah, and thus the methuselah can learn from the neonate (or mortal) in turn, and a Mithraic ghoul who can operate during daylight without expending blood on keeping themself safe while doing so can help protect the vampires from daytime threats (or exposure by uninitiated mortal eyes). We all protect and help each other with our own abilities and talents.