This is where I think your getting. If a person were to say in supplication: "O Allah! I ask you alone by Your noble Prophet! (or the name of other righteous persons)".

Br Abuz Zubair of IA had written about this:

5) To call upon Allah alone, asking Him by His Prophet is a valid difference of opinion in Fiqh where none is censured. For example: O Allah! I ask you alone by Your noble Prophet!

This type of tawassul does not entail Shirk by agreement, but it is, nevertheless, a bida'i tawassul, over which the scholars have differed.

Bida’i tawassul is, as al-‘Allama Hasan al-Shatti al-Dimashqi al-Hanbali says while commenting on Matalib Ulin-Nuha, quoting Ibn ‘al-Imad al-Hanbali: ‘Tawassul through the righteous is for one to say: O Allah! I make tawassul to you through your Prophet Muhammad SallAllahu ‘alaihi wa-sallam, or someone else, that you fulfil my need’

Take note, that the tawassul referred to here involves directly calling upon Allah and addressing Him alone, by the right of His creation. It does not involve calling upon anyone other than Allah, for that will be dealt with later on.

This type of Tawassul is differed over amongst the scholars, including the Hanbalis.

Some scholars, including Ibn Qudama allow this type of Tawassul, while other scholars, such as Ibn Taymiyya do not allow it.

[...]

And as I quoted before, great Scholars like Imam Abu Hanifa detested this practice. He said:

‘It is detested for the supplicator to say, `I ask You by the right of so and so,' or,`By the right of Your Prophets and Messengers, and by the right of the Sacred House and the Sacred Sanctuary."

Sharhul-'Aqeedatut-Tahaawiyyah (p.234) and Ithaafus-Saadaatul Mustaqeem (2/285) and Sharhul Fiqhil-Akbar (p. 198) of al-Qaaree.
In the above scenario, the supplication is being made to Allaah alone and not the Prophet or saints, therefore it is not shirk.

But when one starts to believe that he can ask/call out to other than Allaah whilst at the same time believing that Allaah is the one who will answer his supplication and the one he is calling out to is nothing more than an intermediary, it is still shirk. This is because the Arabs of the time of ignorance believed that Allaah was the Lord, but they commited association in worship, i.e. they took their idols as intercessors (and these idols were statues of former righteous people) thus they commited polythiesm.

This is confirmed in the Qur'an:

And they worship besides Allah things that hurt them not, nor profit them, and they say: "These are our intercessors with Allah." Say: "Do you inform Allah of that which He knows not in the heavens and on the earth?" Glorified and Exalted be He above all that which they associate as partners with Him!
[10:18]

Similarly, Allah said of the pagans: ‘Most of them do not believe in Allah, except that they associate partners unto Him’

al-Tabari says in his tafseer: Their belief in Allah is their saying: Allah is our Creator, our Provider, who gives us death and gives us life; while their Shirk is to attribute partners unto Allah in His worship and invocation.

Lastly, Ibn al-Qayyim says in Madarij al-Salikin (1/332):

“From the forms (of Shirk): Requesting the dead for needs, seeking their aid and turning to them.

This is the basis for Shirk in the world. This is because the actions of the dead have ceased. He is not able to harm or benefit himself, let alone the one who seeks his aid, or asks him to fulfil his need, or asks him to intercede for him with Allah, for this is from his ignorance with respect to the intercessor and the one interceded for, as has preceded. This is because he (the intercessor) is not able to intercede for him with Allah, except with His permission. Allah did not make his ‘seeking aid’ and petitioning, a cause for Allah’s permission to be granted. The only cause that grants Allah’s permission (for intercession) is the perfection of Tawheed. Yet, this Mushrik comes along, with a cause that only prevents Allah’s permission (for intercession)!”

I think you'll get more clarification in these posts:
http://www.islamicboard.com/691705-post4.html (All about Istighatha, Istishfa', and Tawassul, etc...)
http://www.islamicboard.com/691707-post5.html (All about Istighatha, Istishfa', and Tawassul, etc...)
http://www.islamicboard.com/691708-post6.html (All about Istighatha, Istishfa', and Tawassul, etc...)

I know your origninal question was a bit different to this, but I figured that if you read all the possibilities, it might be more clearer to you. Besides, this isn't offtopic Ask if you have any more questions