Some states, specifically Vermont, New York, and Virginia, did so because they were concerned that the US constitution does not provide a mechanism to allow them to withdraw unilaterally.
If state ratification declarations are the legal mechanism for secession, then the first seven states, (with no withdrawal language in their ratification declarations), withdrew illegally. Only Virginia, which withdrew later would have done so without violation of the US constitution.
Are you saying that each state entered the union under different and distinct terms as determined by their ratification declarations?
The states, as sovereign entities, created the federal government as their servant - not their master. They each seceded from the existing Articles of Confederation & Perptual Union, their right to do so stated by Madison in Federalist 43: "the great principle of self- preservation; to the transcendent law of nature and of nature's God, which declares that the safety and happiness of society are the objects at which all political institutions aim, and to which all such institutions must be sacrificed. ... The time has been when it was incumbent on us all to veil the ideas which this paragraph exhibits. The scene is now changed, and with it the part which the same motives dictate."
In other words, while the right of secession was "veiled", the unwritten right still trumphed the legal requirements. Implying that one group of people cannot bind a future group by their actions (which Congress follows even today - one Congress cannot pass legislation binding another).
I would like to point out that those conditions were accepted, and therefore validated as correct principles. The union was not a spider's web one fell into by deceit and became trapped unto death. The acceptance of those declarations stating the right to reassumption of powers was part of the agreement.
If state ratification declarations are the legal mechanism for secession, then the first seven states, (with no withdrawal language in their ratification declarations), withdrew illegally. Only Virginia, which withdrew later would have done so without violation of the US constitution.
They are not the "legal mechanisms" for secession. The secession conventions held by each State were the legal mechanisms by which they decided whether or not to excercise their right to reassume those powers, that is, to secede. Whether or not to reassume those powers was the individual decision of each State, and one of the rights reserved unto it. The form of each State's convention was based on that State's pre-existing format for public referendums to determine the will of the people. After that, if it was determined to be the will of the people, it would be addressed to the union as a declaration, the same as when the State joined. No action was necessary on the part of the union. The ratification declarations are legal testimony to the existence of that right and it's acceptance by the framers. They are the documents that created the union. If that right existed for one, it existed for all.
Are you saying that each state entered the union under different and distinct terms as determined by their ratification declarations?
Absolutely not. That some States included that language in their ratification declarations only proves that it was an accepted idea at the formation of the union, and that States had the right to reassume those powers. If one could, they all could. If not, the framers would not have accepted and allowed those conditions to become part of the documents that created the union. It is worth noting that Vermont was not one of the original 13 and joined afterwards. Even after the original formation of the union, the right to reassume those powers was recognized.
I'm not sure I understand your argument as it applies to Texas. Texas voters elected representatives to a secession convention endorsed by the state legislature. The elected delegates to the secession convention then voted to secede, subject to confirmation by the voters of the state themselves. The voters of the state then overwhelmingly voted to secede.
The Texas procedure used the same steps that the original states did to ratify the US Constitution and, in fact, went even further by submitting the secession question directly to the voters themselves.
We the delegates of the people of Texas, in Convention assembled, have passed an ordinance dissolving all political connection with the government of the United States of America and the people thereof and confidently appeal to the intelligence and patriotism of the freeman of Texas to ratify the same at the ballot box, on the 23rd day of the present month.
But perhaps you are quibbling with the words used in the Texas secession documents. Perhaps the words didn't say "withdraw from the Constitution". If that is your complaint, then you are quibbling and splitting hairs. The Texas documents said that the 1845 act of the people of Texas by which Texas joined the Union was repealed and annulled and that Texas sovereignty was resumed.
Texas would seem to be following the guidance of James Madison, father of the US Constitution, who said:
It appears to your committee to be a plain principle, founded in common sense, illustrated by common practice, and essential to the nature of compacts, that, where resort can be had to no tribunal, superior to the authority of the parties, the parties themselves must be the rightful judges in the last resort, whether the bargain made has been pursued or violated. The Constitution of the United States was formed by the sanction of the states, given by each in its sovereign capacity. It adds to the stability and dignity, as well as to the authority of the Constitution, that it rests on this legitimate and solid foundation. The states, then, being the parties to the constitutional compact, and in their sovereign capacity, it follows of necessity, that there can be no tribunal above their authority, to decide in the last resort, whether the compact made by them be violated; and, consequently, that, as the. parties to it, they must themselves decide, in the last resort, such questions as may be of sufficient magnitude to require their interposition.