When converting a preexisting enemy stat block, I use the following CR-based formula for converting any resistances and vulnerabilities already on the sheet:

CR	Resistance/Vulnerability level
0-4	1
5-9	2
10-14	3
15-19	4
20+	5

If you deem the enemy to be especially weak or strong against a damage type, you can increase this value by 1 (or more, at your discretion).

• A shadow for example might warrant vulnerability 2 to radiant damage, despite only being CR 1/5.

While not every creature needs vulnerabilities, I recommend that most have vulnerability 1 to a damage type. This will give players something to look for in combat, and let them feel rewarded when they figure out the vulnerability.

• Unless that creature is meant to be especially vulnerable to said damage type, keeping the vulnerability at 1 is recommended.
• You can use these minor vulnerabilities to represent small interactions between damage types and the creature, narratively. A knight in full plate armor might be vulnerable 1 to lightning as it conducts through his armor, a plant creature might be vulnerable 1 to fire as it burns, and so on.