Actually you do not need to kill the ophidians to finish this quest. It will take 10 crystals if you've got them, the ophidian part is just in case you don't have 10 crystals. No bug here as you were still probably charged 10 crystals.

Similar to the artic wolf cloak guy, since all he really needs are the pelts, it doens't have to be from the wolves he spawned for you.

Did you have 10 cystals in your resources. If you have 10 crystals then you can finish the quest as the quest will take those crystals from you. If you are saying you didn't have 10 crystals and you were able to advance?

Edit: Did a test, if you do not have 10 crystals it doesn't say anything when you revisit the location, but if you have 10 crystals then it removes them from your resources as it should.

The quest could be updated to tell you to get the crystals when you return without them.

Well I'm out of ideas as to why you were able to run the quest forward without having the 10 crystals as your resources (you do not need to get them from the Ophidians) Here is the quest condition, it cannot advance without charging the crystal. 

<QuestConditionDef>
    <Objective>
        <Icon>Mana_32.png</Icon>
        <Text>Gather 10 crystals.</Text>
        <IsOptional>0</IsOptional>
    </Objective>
    <Class>Success</Class>
    <Type>CheckForResource</Type>
    <TextData>RefinedCrystal</TextData>
    <NumericData>10</NumericData>
</QuestConditionDef>

The oddity of this quest is that you do not have to revisit the quest location to continue the quest. Another condition is required to force that to happen similar to the Arctic Wolf cloak quest.

If one edits the quest we could force the player to return back to quest location. But, it still charges the 10 crystal by the above condition.

<QuestConditionDef>
    <ID>1</ID>
    <DependentCondition>0</DependentCondition>
    <Objective>
        <Icon>Mana_32.png</Icon>
        <Text>Bring 10 crystals to Vatula.</Text>
        <IsOptional>0</IsOptional>
    </Objective>
    <Class>Success</Class>
    <Type>UnitEntersQuestLocation</Type>
    <CannotCompleteText>I need the crystals to find the Cloak of Stars.</CannotCompleteText>
</QuestConditionDef>

N/A

Fixed (or improved), thanks Parrotmath!