Here is my interpretation of those lines.
To understand this poem, you have to know the myth of Trisan and Iseult. There are a number of versions, but most of them go as follows:
Tristan takes Iseult to marry his uncle, King Mark. On the way there, somebody gives them both a love potion and they fall hopelessly in love. Iseult marries Tristan's uncle anyway. In many of the original versions, some tragedy follows, but in this version, it seems the two lovers get away with it: they manage to keep their love affair secret:
None, unless the saints above,
Knew the secret of their love;
For with calm and stately grace
Isolde held her queenly place,
The poem says that ordinary love, that hasn't met with any obstacles, will fade over time:
For love will grow wan and cold,
And die ere ever it is old,
That is never assailed by fears,
Or steeped in repentant tears,
Or passed through the fire like gold.
However, in the poem Tristan and Isolde's love doesn't fade (maybe because it was caused by a love potion, as in the original story, but more likely because it is confronted with insurmountable obstacles). However, the necessity of keeping it secret causes them pain, anguish, and despair:
Little dreaming hours would come,
Like pale shadows from the tomb,
When an open death of doom.
Had been still less hard to bear,
Than the ghastly, cold despair
Of those hidden vows, whose smart
Pale the cheek, and break the heart.
So the poem is saying that maybe the lovers would have been happier if they had met an "open death of doom", as in some of the versions of the legend, where Tristan's uncle discovers their affair and kills Tristan.
The "hidden vows" in the second-to-last line are the secret promises that Tristan and Isolde made to each other.