#define in C, legal character There is a C structure
struct a
{
int val1,val2;
}

I have made changes to the code like
struct b
{ 
int val2;
}
struct a
{
int val1;
struct b b_obj;
}

Now, usage of val2 in the other C files is like a_obj->val2;.
I want to replace its declaration usage and there are a lot of them, so I have defined a macro in the header file where the struct a is defined as follows:
#define a_obj->val2 (a_obj->b_obj.val2)

It's not working. Is -> illegal in the identifier part of a macro definition #define?
Could someone please tell me where am I wrong?
Edit as suggested by @Basile -
It's a legacy source code, a very huge project. Not sure of LOC.
I want to make such changes because I want to make it more modular.
For example I want to group similar fields of the structure under a same name and that's the reason I want to create another struct B with fields which are related to B feature and also common to A.
I can't use Find Replace feature of other text editors, I am using VIM.

This kind of macro magic will get you into trouble soon,
because it is making your source code unreadable and brittle (credits Basile for the phrasing).
But this should work for what you describe.
struct b
{ 
int val2m;
}
struct a
{
int val1;
struct b b_obj;
}

#define val2 b_obj.val2m

The trick is to give the actual identifier inside the struct declaration a new name (val2m), so that the name all the other code uses can be turned into a magic alias,
which then can contain the modified access to take a detour via the additionally introduced inner struct.
This is only a kind of band-aid for the problematic situation of having to change something backstage in existing code with many references. Only use it if there is no chance of refactoring the code cleanly. ("band-aid", appropriate image by StoryTeller, credits).
I explicitly recommend looking at Basiles answer, for a cleaner more "future-proof" way. It is the way to go to avoid the trouble I predict with using this macro magic. Use it if you are not forced by very good reasons.

As other explained, the preprocessor works only on tokens, and you can only #define a name. Read the documentation of cpp and the C11 standard n1570.
What you want to do is very ugly (and there are few occasions where it is worthwhile). It makes your code messy, unreadable, and brittle.
Learn to use better your source code editor (you probably have some interactive replace, or interactive replace with regexp-s; if you don't, switch to a better editor like GNU emacs or vim - and study the documentation of your editor). You could also use scripting tools like ed, sed, grep, awk etc... to help you in doing those replacements.
In a small project, replacing relevant occurrences of ->val2 (or .val2) with ->b_obj.val2 (or .b_obj.val2) is really easy, even if you have a hundred of them. And that keeps your code readable. Don't forget to use some version control system (to keep both old and new versions of your code).
In a large project of at least a million of lines of source code, you might ask how to find every occurrence of field usage of val2 for a given type (but you should probably name val2 well enough to have most occurrences of it be relevant; in other words, take care of the naming of your fields). That is a very different question (e.g. you could write some GCC plugin to find such occurrences and help you in replacing the relevant ones).
If you are refactoring an old and large legacy code, you need to be sure to keep it readable, and you don't want fancy macro tricks. For example, you might add some static inline function to access that field. And it could be then worthwhile to use some better tools (e.g. a compiler plugin, some kind of C parser, etc...) to help you in that refactoring.
Keep the source code readable by human developers. Otherwise, you are shooting yourself in the foot. What you want to do is unreasonable, it decreases the readability of the code base.

I can't use Find Replace feature of other text editors, I am using VIM.

vim is scriptable (e.g. in lua) and accepts plugins (so if interactive replace is not enough, consider writing some vim plugin or script to help you), and has powerful find-replace-regexp facilities. You might also use some combination of scripts to help you. In many cases they are enough. If they are not, you should explain why.
Also, you could temporarily replace the val2 field of struct a with a unique name like val2_3TYRxW1PuK7 (or whatever is appropriate, making some unique "random-looking" name is easy). Then you run your full build (e.g. after some make clean). The compiler would emit error messages for every place where you need to replace val2 used as a field of struct a (but won't mind for any other occurrence of the val2 name used for some other purpose). That could help you a lot -once you have corrected your code to get rid of all errors- (especially when combined with some editor scripting) because then you just need to replace val2_3TYRxW1PuK7 with b_obj.val2 everywhere.


Is -> illegal in #define?

Yes.
#define identifier can only be letter, number or underscore.

Macros definitions must be regular identifiers, so you can't use any special character like - or >.
I've thinked that may be you can use an union, like this:
struct b
{
    int val2;
}
struct a
{
    int val1;
    union {
struct b b_obj;
int val2;
    }
} 

so you can still using a_obj->val2.