Preempting vulnerable should be "sound". It is unnecessary to be too aggressive at this vulnerability, the focus should be on quality hands. One big bonus to this is, when you open say 3♠ , your partner can confidently raise to 4S and expect to make it, with a moderate hand.
Key considerations when preempting vulnerable
Suit quality - Suit quality is paramount. 10's and 9's are critical filler cards.
This suit is not good enough for a vulnerable weak 2.
This suit is still not really good enough, though some might scrape over the line with it depending on the rest of the hand
This is a good example of a sound weak 2.
Similarly, this is also acceptable. Partner can gauge your trick count well when he is looking at the ♠ K, knowing it will probably come to 6 tricks.
Suit quality summary - 2 of the top 3 honors is generally needed for a vulnerable preempt. 10's and 9's are also very important cards, it may not be a great idea to open without those cards.
A top US expert recently opened 2♠ with this suit, as it lacked intermediate cards too much, he was not willing to open on the three level.
Second consideration - shape. 6322 or 7222 is very bad shape to preempt on. Its typically a bad idea to preempt on these shapes. 6331, or 6421 are much better offensive shapes, your chances of making contracts skyrocket. 7321 should generally be the minimum shape for a three level vulnerable preempt.
This is a respectable 2 ♠ opening
This would not be adequate for a 3♠ vulnerable opening, due to the 7222 shape. 2S would have to suffice.
Final consideration - cards in the majors. If you have good holdings in the majors, its generally not a good idea to preempt. Good holdings include three decent cards. For example
You should notice two things about this hand. The suit quality isn't good enough. But also, your spades are too good. The idea is that, you may preempt and partner has one card in your suit, while holding 5 card spades. Spades may be a significantly better contract. Even if the heart suit quality was good enough, its advisable not to preempt with three good cards in the other major. It may affect your chance of bidding a making vulnerable game.
When preempting Non-Vul, the focus is much more on being destructive. Taking up space and making it difficult for the opponents. There is less requirement on suit quality. Partner should not raise you unless they hold an excellent hand, because its not reasonable to expect a great hand from a non-vul preempt.
In a recent american national, an expert player had no problems opening 3♠
So clearly the standards are not very high with suit quality, however the hand is unbalanced which is a big positive (7321 is nice, 7222 is considerably worse and not always worth bidding).
Holdings in the majors are still very important, particularly in spades.
It would not be a great idea to open this hand 3 ♦ , as the hand is excellent if partner bids spades, you would have good chances of making game opposite a 1♠ opener.
This hand is another poor choice to open,
The hand is too good if partner has a major - you have support, a singleton, a nice side suit etc. Your hand just looks too good for dummy duty.
Keeping the shape the same, but the texture different, this hand is now a decent 2♦ weak opening. As this time the hand has its emphasis in the quality diamond suit.
overall, when your side is Not vulnerable, you should preempt more boldly and aggressively. Suit quality is less of a requirement than it was vulnerable. Nevertheless, there are still sometimes where it is a good idea to hold back, particularly when you have reasonable holdings in the major(s).