Sonnet XXVI

Lord of my love, to whom in vassalage
Thy merit hath my duty strongly knit,
To thee I send this written embassage,
To witness duty, not to show my wit:
Duty so great, which wit so poor as mine
May make seem bare, in wanting words to show it,
But that I hope some good conceit of thine
In thy soul's thought, all naked, will bestow it:
Till whatsoever star that guides my moving,
Points on me graciously with fair aspect,
And puts apparel on my tottered loving,
To show me worthy of thy sweet respect:
   Then may I dare to boast how I do love thee;
   Till then, not show my head where thou mayst prove me.

The 1609 Quarto Version

LOrd of my loue,to whome in vaſſalage
Thy merrit hath my dutie ſtrongly knit;
To thee I ſend this written ambaſſage
To witneſſe duty, not to ſhew my wit.
Duty ſo great, which wit ſo poore as mine
May make ſeeme bare,in wanting words to ſhew it;
But that I hope ſome good conceipt of thine
In thy ſoules thought( all naked ) will beſtow it:
Til whatſoeuer ſtar that guides my mouing,
Points on me gratiouſly with faire aſpect,
And puts apparrell on my tottered louing,
To ſhow me worthy of their ſweet reſpect,
  Then may I dare to boaſt how I doe loue thee,
  Til then,not ſhow my head where thou maiſt proue                                                                                      me.

Commentary

1. Lord of my love, to whom in vassalage

See the comment above. A vassal was a lowly person in the employ or service of a great lord. The word had feudal associations, suggestive of serfdom. vassalage is the duty and service of the vassal. Lord is possibly a reference to the title of the addressee, as champions of Pembroke or Southampton would infer. It is, however, just as likely to be a general term invoking the ideas of majesty of the beloved (in the eyes of the lover) and its concomitant vassalage.

2. Thy merit hath my duty strongly knit,

Your merit ensures that I perform my duty of serving you without question;
strongly knit = tightly bound (to serving you).

3. To thee I send this written embassage,

embassage = a message brought by an ambassador. This sonnet itself could be conceived as an ambassador carrying a message. Q's spelling of the word is ambassage. Compare also 

In tender embassy of love to thee, 45

and see the note thereon - (Sonnet 45).

4. To witness duty, not to show my wit:

wit = creative ability, invention, intellectual talents; wisdom. (OED.1, 2.a., 6.a.). The application to 'humour' or 'sense of humour' is somewhat later, although 'cleverness of thinking' almost implies the ability to make humorous connections between things. The repetition of wit in this line (first as part of witness) and then in the complementary words duty and wit in the following line, are rhetorical flourishes worthy of an accomplished orator. See KDJ.Sonn.p.162.nn2-5,4. for the Greek rhetorical names.

5. Duty so great, which wit so poor as mine

wit = quickness of intellect, understanding, intelligence. See note above.

6. May make seem bare, in wanting words to show it,

My lack of intelligence may make the duty and this written embassage seem bare and meaningless, through being unable to supply adequate words to show its meaning and purport.

7. But that I hope some good conceit of thine

But that I hope = nevertheless I retain a hope that;
some good conceit of thine = some imaginative inspiration on your part.

8. In thy soul's thought, all naked, will bestow it:

In thy soul's thought = deep in your heart;
all naked - refers back to duty, line 5, which his poor wit makes seem bare;
will bestow it = will stow it away, until etc. Also with a suggestion of bestowing riches upon it; consequently, will redeem its unworthiness.

9. Till whatsoever star that guides my moving,

Until whatever star it is that directs my life. The stars were thought to emit invisible fluids which affected the lives of humans.

10. Points on me graciously with fair aspect,

Points on me = Looks down on me, points its rays at me;
with fair aspect - The aspects and conjunctions of stars and planets were important considerations in astrology. When the conjunctions were in certain favourable states, the planets' aspects were then said to be favourable. Planets were regarded as wandering stars, and were not strictly separated from stars proper, as they are in modern astronomy. Hence the line paraphrases as 'Looks on me with a benign and gracious blessing, foretelling good fortune'.

11. And puts apparel on my tottered loving,

The duty and written embassage, which were shown to be naked and bare, are now supplanted by loving, which is also tattered (tottered being a frequent spelling) and in rags, and needs to be clothed by a favourable star, which puts apparel (clothing) on it.

12. To show me worthy of thy sweet respect:

So that I may become worthy of your attention (and love).
respect = consideration, care, esteem.

13. Then may I dare to boast how I do love thee;

Then = when I have achieved the improvements in fortune listed above.

14. Till then, not show my head where thou mayst prove me.

not show my head - I am inclined to think that this is a an image from warfare, as in 'Do not show your head above the parapet' rather than the proverbial 'He dare not show his head for debt'. There is inherent danger in loving above one's social station.
where thou may'st prove me = where you may put me to the test (by the awkwardness of the situation). Or, where you may cause me to show how true and resolute my love is.